A number of fights occur in arenas where it would be possible to move back through the entrance and cheese the boss by ignoring mechanics.

Take the Abomination in ICP for instance. All mechanics in the fight have set locations in the arena. Thus if the doors to the arena did not close during the fight, it would be trivial to move outside where the boss has no chance to hit you, and take him down with ranged abilities.

We saw this exact thing happen with the final boss in Tempest Island, where you could kite the boss down the entrance ramp, and go through the entire fight without having to deal with the whirlwinds.

Tl;dr rooms tend to lock to avoid cheese tactics, because it is easier than making tethering mechanics.

Alot of bosses are too big to make it through the doors. I would guess it's so you can't stay outside the door and beat him down without him touching you...

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It's just there to prevent exploiting. A lot of the bosses can be either pulled out of their rooms(at which point they might stop doing some of their room-centered mechanics) or agroed from outside the room at which point you can safely range attack them to death with most of their attacks never reaching you.

Some of those need fixing, like doors on Engine Guardian fight are a giant pain in...certain places because boss goes down soon as first person enters the room and then anyone going through the room will initiate combat and get doors closed, even if there're still people behind it. The key is either everyone goes in at once or everyone sneaks, but it's super unintuitive and hard to know for new players. Others, like the Maw boss in CoA II, I'm just so glad you can't exploit it anymore...of course, now you can solo that boss with closed eyes pretty much so not like that matters, but still I HATED people pulling him out of his room so they wouldn't have to deal with ads. I wouldn't mind Overfiend door being fixed as well because I hate doing the gate stack thing yet everyone seems to firmly believe that's the only way to do it.