Group Selection

Anyone know why GM's and group posters are so selective about party members?  I've been playing D&D and pathfinder for years (never on roll20 until recently) and the only thing I've ever encountered are so loose rules about not working against the party or about having some diversity in ability.  Here on roll20 it looks like everyone is trying to optimize and custom build their campaigns to desired specs-which seems completely contradictory to the desired suspension of disbelief you can achieve with this wonderful game.  I take no personal issue with this - just curious if my in-person experiences have missed something that the online community finds important.
Well, hmm i try to explain. I don't do it myself but i understand why. See there's a lot more players here than gm's even less gm's if you do not count paid gming aka wanna play for free. And not everyone is a good fit with certain people not saying there bad people but personalities can conflict. and online is harder to judge than face to face. and the gm's want a lasting game where everyone gets along and have fun and that's the point of playing. so they have to be picky to a degree to insure this. I hope this helps