Why do people say nuclear fusion is 50 years away? This '50 years' number is floating around media pretty consistently, even the ITER road map claims that the first commercial reactor will happen post-2050. But why is that? I understand that there is some homework to do before fusion can go commercial, but is there any reason to believe that the R&D time does not simply scale with the amount of resources invested? 
EDIT: In plain words, what are the assumptions that went into this '50 years' estimate? (invested manhours, computer hours, etc.)
 A: Maybe this doesn't answer the question (maybe it does), but then you may include bits of the below in the question (as a reference). Or read the full report first (not just the Summary).

It is estimated that up to the point of possible implementation of electricity generation by nuclear fusion, R&D will need further promotion totalling around €60–80 billion over a period of 50 years or so (of which €20–30 billion within the EU) based on a report from 2002.

Wikipedia

Fusion experiments are becoming increasingly large-scale with a high degree of technical complexity, requiring substantial financial investment. In the light of these framework conditions, international cooperation is particularly intensive and stable. The scale of resources needed and very long period to possible imple- mentation, with the resulting extremely great uncertainties in evaluation lead to major complexity in the pending decisions.
The community of fusion researchers believes that the reactor-oriented research programme should be continued with two intermediate phases – ITER (Inter- national Thermonuclear Experimental Reactor) and DEMO (Demonstration Fusion Powerplant) – to prepare for construction of the first commercial fusion reactor in around 2050. ITER, which currently requires far-reaching decisions, is a partnership between the EU, Japan and Russia, with other states involved. In parallel to ITER, construction of a special high-intensity fusion neutron source is needed to develop and test low activation materials. DEMO is intended to demonstrate the technical feasibility of a fusion power plant and generate electricity in continuous operation for the first time.
To achieve this programme, very substantial scientific and technical challenges must be mastered. The R&D process required will take several decades and promotional funding on a large scale. In the almost 50-year history of fusion research, the difficulties in developing a fusion power plant were repearedly [sic] underestimated, with the result that the horizon for implementation had to be pushed further and further into the future, becoming in effect a »moving target«.
Nuclear fusion is also a particular challenge for technology assessment. Forecasts of the technological impacts of fusion in more than 50 years are extraordinarily
difficult, and require careful interpretation. They are generally no more than heuristic approaches which might give some indication of what requires special attention in the further development process of fusion. The assessment is methodologically complicated by the fact that the quality of the numbers supplied by fusion research is very difficult to judge, given the possible wishful thinking involved and the impossibility of finding »independent« know how.

Summary of the referenced report
A: From the ITER page

The experimental campaign that will be carried out at ITER is crucial to advancing fusion science and preparing the way for the fusion power plants of tomorrow.
ITER will be the first fusion device to produce net energy. ITER will be the first fusion device to maintain fusion for long periods of time. And ITER will be the first fusion device to test the integrated technologies, materials, and physics regimes necessary for the commercial production of fusion-based electricity.
Thousands of engineers and scientists have contributed to the design of ITER since the idea for an international joint experiment in fusion was first launched in 1985. The ITER Members—China, the European Union, India, Japan, Korea, Russia and the United States—are now engaged in a 35-year collaboration to build and operate the ITER experimental device, and together bring fusion to the point where a demonstration fusion reactor can be designed.

Just contemplate :

The successful integration and assembly of over one million components (ten million parts), built in the ITER Members' factories around the world and delivered to the ITER site constitutes a tremendous logistics and engineering challenge. The assembly workforce, both at ITER and in the Domestic Agencies, will reach 2,000 people at the height of assembly activities. In the ITER offices around the world, the exact sequence of assembly events has been carefully orchestrated and coordinated beginning with the arrival of the first large components on the ITER site in 2015.

And this will  

1) Produce 500 MW of fusion power
The world record for fusion power is held by the European tokamak JET. In 1997, JET produced 16 MW of fusion power from a total input power of 24 MW (Q=0.67). ITER is designed to produce a ten-fold return on energy (Q=10), or 500 MW of fusion power from 50 MW of input power. ITER will not capture the energy it produces as electricity, but—as first of all fusion experiments in history to produce net energy gain—it will prepare the way for the machine that can.
2) Demonstrate the integrated operation of technologies for a fusion power plant
ITER will bridge the gap between today's smaller-scale experimental fusion devices and the demonstration fusion power plants of the future. Scientists will be able to study plasmas under conditions similar to those expected in a future power plant and test technologies such as heating, control, diagnostics, cryogenics and remote maintenance.
3) Achieve a deuterium-tritium plasma in which the reaction is sustained through internal heating
Fusion research today is at the threshold of exploring a "burning plasma"—one in which the heat from the fusion reaction is confined within the plasma efficiently enough for the reaction to be sustained for a long duration. Scientists are confident that the plasmas in ITER will not only produce much more fusion energy, but will remain stable for longer periods of time.
4) Test tritium breeding
One of the missions for the later stages of ITER operation is to demonstrate the feasibility of producing tritium within the vacuum vessel. The world supply of tritium (used with deuterium to fuel the fusion reaction) is not sufficient to cover the needs of future power plants. ITER will provide a unique opportunity to test mockup in-vessel tritium breeding blankets in a real fusion environment.
5) Demonstrate the safety characteristics of a fusion device
ITER achieved an important landmark in fusion history when, in 2012, the ITER Organization 
  was licensed as a nuclear operator in France based on the rigorous and impartial examination of its safety files. One of the primary goals of ITER operation is to demonstrate the control of the plasma and the fusion reactions with negligible consequences to the environment.

The  construction of a commercial reactor  is a very complex engineering project, and that is why the commercial date is put off 50 years in the future. All people working now on ITER will be retired :). 
Edit : I asked a physicist working on ITER what are  the delays
The building for the housing of the reactor  started in 2010. There are continuous stops due to nuclear safety checks , i.e. that the building will last the projected years  under the intense radiation.
He believes it is mostly bureaucracy. All decisions go to committees with representatives of participating institutes,  plus administration is the usual EU bureaucracy.  
It seems that it follows a modern greek proverb:
Where too many roosters crow, dawn is delayed .
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