most of the people that willingly add up to medic (myself included) are people who dont even actually want to play medic, but wouldn't get to play a pug on the website ever otherwise since the same handful of people just play every pug. thus you get lower quality medics (again myself included lol) and often people get annoyed or mad having to play with lower quality medics and then make the pug less fun for the medic player and encourages them not to readd because they'd have to deal with that player or just that general unfun vibe in the pug again
OR sometimes the added medics are established players but still also dont main medic, and they only add up so that the pug will even start and thus their heart isn't in it and just has to entertain themselves or check out and play on autopilot
medic itself, id also wager to guess for most people who browse the competitive TF2 forum, is just not fun to play in 6s. you are a walking ult timer with little to no say on what actually happens in the game and you don't do anything but follow people around. at least overwatch drip feeds mini dopamine hits with abilities like ana nade/sleep and all the other shit that supports get to do besides heal. shooting arrows is fun but then your team will just get mad at you (understandably if you're trying to win tbh) for arrowing too much so you just have to be walking fleshlight all game