Why are there no free quarks? I am not looking for "because you can't pull them apart". If you have a problem understanding the question, then understand it as: Were there ever free quarks in the past? How did they get to their current unfree state?
The force that keeps quarks together gets stronger the further you try to pull them apart. This means that you need to use more energy the further you want to pull them apart. Eventually there is enough energy in the system to produce a new pair of quarks (quark plus anti-quark). These two new quarks then respectively join up with the quarks that tried to pull apart. So you end up with separate mesons or baryons, but no separate quarks.