Because you yourself can't be good with the UMP, because it takes no skill to use. Everyone is good with that gun.
0 recoil, 2SK with SP, low-ish fire rate keeps recoil down (who cares anyway, kills in 2 hits) and easy sight make it a gun with literally no downside. It would only ever lose in CQB to the MP5K anyway, and maybe the TAR.
Tell that to any pro who's tried to become a god at the F2000. It just IS NOT possible.