You haven't been here for long - this has always happened to an extent. Most players who are looking to clear just want the party that gives them the least risk, and tierpicking/locking out jobs is a means to an end in that regard. Healers are close enough together, though where I grew up there was healer lockout happening for WHM to an extent due to AST being so much more beneficial than it. MCH would get locked out in favor of bard because it synergized much better with other jobs, etc.
Shadowbringers alleviated this to an extent by deleting job synergy completely, but it never *really* went away. There will always be players locking others out based on job because of the leeway other jobs will give them due to just being straight up better.
At the end of the day, this is inherently a numbers game. I don't know if you've played other RPGs before, but in other RPGs you also tend to pick the guy with the better stats over the one you need to work harder with. Because if you hit the 0.1% with a MCH, you would've cleared with a DNC or BRD. You can't vet other players coming in (officially speaking), so you don't know what they're capable of. So, the natural response is just to take what's best and leave the rest.
One of my friends I used to raid with was a MCH, and since we were 5 people we could usually get away with it by being able to vouch for them even when they were locked out in overall PF, but I wouldn't want to be a solo MCH in PF right now.
It's a problem the devs sought to solve, but never really did because they never did get the balancing right, and at this point it kinda looks like they've just thrown their hands up in the air entirely.