Medic has zero mobility, can’t damage boost unless he sacrifices stock for the Kritzkreig, and can’t res. Uber only lasts 8 seconds, plus he loses all Ubercharge on death unless he’s using the Vita-Saw, which any self-respecting “meta” Medic doesn’t use.
Which brings me to the points that half of Medic’s weapons aren’t viable in competitive, plus he’s only run thanks to Uber and his status as the only primary healer.
Your point about Mercy being boring is quite moot. Medic brings less to the table and doesn’t have freedom of movement to boot.