Why quarks are confined? Why can't they be found in unbound states?

If you want a more descriptive and a simple mathematical treatment using the formulation of electrodynamics and quantum mechanics, then Leonard Susskind's series of lectures will give you a lot of insights!!

Here is the Link : https://inspirehep.net/literature/1532

Quarks that are lighter than the QCD scale, ($sim 200,{rm MeV}$) will always be confined, but heavier ones can be unbound. The top quark weighs $170,{rm GeV}$, so it can be unbound.

The QCD scale is the mass scale that pops out when you renormalize the strong interactions.

This is because the force-versus-distance law for quarks is such that the farther away from one another you pull a pair of quarks, the harder they attract one another. It is as if they were attached to each other with a rubber band (a very stiff one!). If you pull them far enough apart, there's enough energy stored in the system to create a new pair of quarks (i.e., the rubber band snaps) which pair up with the others and instead of getting two "free" quarks, you get a pair of mesons with two quarks inside each one.