Theoretical and Practical Rationality

I understand that some believe that practical rationality is autonomous from theoretical rationality, and that the search for moral realism is a lost cause because it's an attempt to build up practical rationality from theoretical rationality. I sorta understand that. What I don't even sorta understand is why we would try to philosophize about practical rationality, if that is the case. Isn't that the attempt to bring our theoretical reasoning abilities to bear on practical rationality? Yes, to understand something is not to inhabit it, but then how is that different from the original attempt to do moral philosophy from the standpoint of the theoretical reasoner? I'm confused.