Why do smartphones need towers to send messages? Why do smartphones need towers to send messages? Why can't they send messages directly to each other?
 A: The battery on your smart phone only provides approximately 3 W of power to the phone's transmitter.  This is enough power to reach a nearby cell tower, but totally insufficient to send a signal for several hundred miles.  Due to this, the nearest cell tower is used to relay your signal to another cell tower that is located close to the party that you are attempting to call, and that tower completes the call to that party.
A: They can and do, just not commercial phones. Commercial phones were built on a cellular architecture because it's possible, and it provides the greatest capability to support a lot lot of simulataneous traffic with an efficient use of spectrum. Direct wireless device communications is both simpler and more complex. Bluetooth is a simple way, but can not carry Voice or any significant bandwidth message. Software radio using adhocmnetwork networking is the more complex way.  
The cellular system was arranged (designed) that way, cell phones to towers to the POTS (regular inter office network based on cables or fiber) to another tower back to another phone in order to lower the power needed for cell phones (yes, you can call them smart phones, the wireless network simply carries bits, doesn't care if they are smart or just digital voice) to actually nowadays less than a watt, so they could be small. The other reason, and maybe more important, it is a way to reuse frequencies (expensive spectrum): each tower or small group of towers, called clusters, uses the same freq as another group. This is called the frequency reuse factor. Clusters used to have 7 or more cells, nowadays is 3 or 1 mostly. 
In addition areas have multiple cells and multiple clusters, and each cluster reuses the same freqs with other clusters. There are many clusters in a city or area. Frequencies are used over and over in an area by a large factor of maybe 100 or more times. It increase the number of users that can be simultaneously supported inan area, or city. The capacity then depends on the number of cell towers and the reuse factor. Nowadays the reuse factor can be 1 (i.e., every tower can use the same frequencies). Other technologies also enter in, so that so many people can use it at the same time, with the limited spectrum used. It is the most efficient use of spectrum of any technology in the planet.  
Yes, there were some Wilkie talkie type phones, like Nextel. Actually, they also used towers. And now they all do. With exception of real toy walkie talkies. 
These use freqs from about 700 MHz to 3 GHz, and mostly in the 1-2 GHz range. Even with more power they can't go too far. At those freqs in all but rural environments without much vegetation the propagation goes like 1/r^n, with n between 3 and 4 because of diffraction, scattering, reflections, blockages and resulting in a lot of loss and multipath. So those freqs have a hard time with anything longer than a few Kms. In the initial first generation cellular it was the 800-900 MHz range, and in rural areas you could get, sometimes, 10 or 20 Kms. Nowadays mostly you don't. 
But, there are radios built to go further, 30 Kms or more. Those are generally VHF and the lower UHF range. They are built for and used by the military and, Ina different way, first responders. They usually use more power. One of the issues is the protocol for multiple access, and for switching and routing. Where there are no towers, it's an adhocmnetwork sometimes, and the protocols do the routing. They have existed for over 20 years, basically some sort of wireless open shortest path first like in the Internet (except it is much tougher because wireless links are not reliable like the terrestrial Internet). 
Some of the newest (well, 10 years old or more) technology is software radio, where the protocols are adaptive. Military communications uses it. Search software radio. 
BTW, the physical layer waveforms are humongously (i.e., hugely) advanced from FM or AM. They went to digital PSK up to level 128 or 256 (256 possibLe phases, yes, crazy, loTs of efficient error correction), then spread spectrum, nowadays different versions of OFDM (orthogonal freq division multiplexing, basically an FFT and modulating subcarriers with minimal spacing).
There is also sat phones, but the economics is much more expensive, for physical reasons. Ask why if you wish but it's mainly because launching and maintaining satellites is too expensive. That is changing, but cellular is just too cheap. 
So, no, they don't need towers but it is the only way, for now, to make it so cheap that you can waste the wireless spectrum watching nonsense YouTube and Facebook and other videos and media broadcasts on your 'smart phone' and only pay $60 a month. 
It's going to get to be even more in the next generation, called 5G. It will probably include some adhocmnetwork networking, i.e. Talk directly (or use another smart phone as relay, the issue has always been who receives the money then).
I have designed and deployed these things, and teach a graduate level course on it. Very close to physics but nowadays almost too easy with the physical layers being taken for granted and the innovation in the silliest applications people can imagine. 
