>Why? Because they do.
This is a crap reasoning process. Why did I set that guy on fire? Because I did! Why did he deserve it? Because I did it! It's circular to say the least.
At some point, the re-use timer (it's not really a cool down timer, it's specifically a timer to inhibit re-use) was put in to balance khri which were designed and built in an entirely different game; not even DR 2.0, pre-DR-2.0. Khri have been redone a little since then, but the core mechanics are just showing their age. The re-use timer originally balanced a system where thieves didn't have a real resource issue; concentration was plentiful and not used by anything.
Not so much any more. It's used by everything thieves do.
And khri now use/depend on skill instead of stats and circle. No one else gets a hard-lock re-use timer on mundane, day to day, abilities. Mages don't have some restriction on spell use (oh wait, hey mana, you look a lot like concentration in this new paradigm, so let's get that out of the way). Barbs don't either.
Assuming a class has the resource/confound to cast/start/whatever the ability, they can do so. Thieves? Not so much.
If the timer is a hard coded aspect of khri, which can't be changed without basically rewriting the entire system from the ground up, then that should be mentioned and the (not insane) players will drop it for the 3.1 release.