I think the better question is that why do some guns without QLA load and shoot every bit as good as the best of TCs?

Many find it convenient, and it is so long as it is done correctly. For my own part, I will always believe it to be an unnecessary addition of a possible way to screw up an otherwise good barrel. The deeper it is made, the easier it is to screw it up.

And I follow Sabotloader around making such blanket and rigid statements just to get his goat.