The biggest reason would be that the people of Maryland oppose retrocession. [1] You cannot make Maryland take back land they do not want without something like an amendment to the constitution.
Each year DC pays more federal taxes per capita than every other state. In raw dollars it is more than 22 other states. [2] The people of DC want representation to go along with that taxation.
I imagine it's possible, but "no reason" is definitely a stretch. The parts of DC that are federal land are hardly contiguous, and trying to force a clean split between DC and Maryland looks like it would be a nightmare: https://www.washingtonpost.com/graphics/local/dc-marijuana-m...
you wouldn't have to be perfect. You could carve out just the contiguous core. I believe this was floated by Jeff Flake. Federal buildings can, in general, exist on state land.
