If they did that, there would likely be a boss there. Being open world, ANYONE could attack the boss. There'd be two ways to handle a boss spawn: the quest way and the hunt way. The quest way would limit the attacks to only those in the party for that step of the dungeon, similar to a levequest and other people being unable to attack the things for you unless you're in a party (although anyone can still heal you). The hunt way would be a timed spawn and would 'require' a large amount of people to get down there in time to kill it and claim credit. Obviously, people would complain about 'early pullers' then. They could make it a short spawn, but to balance it they'd have to give it rather crappy rewards, or limit it to once a week or something, which makes it a chore because then you HAVE to get it before reset or you lose out on it. They could make it take a few days to respawn, with the possibility for very nice rewards, but then some people will miss out because they're not online or it was 'early pulled'. They could also make it spawn only at specific times, so everyone has a chance to get there in time or logging in for it, but that leaves the likelihood of people prioritizing it over real life things ('sorry honey, I'm canceling our date tonight so I can kill this boss!').

I like the idea, but I can't help feeling that somehow it would be ruined by the community.