SOME girls don't bitch that much. However most do. Some bitch so much you wan't to mute them with a brick.
Personally i think it's beacause they're so insecure with themselves that they have to bolster their ego by slagging off everyone who could be slight competetion to them.
Remember guys do it too, they just do it face to face.
I think its only the girls who do it to excess about everyone who are insecure. Ive only ever done it when something about someone has genuinely pissed me off and Ive felt the need to say something
Not all girls bitch it's usually a certain type, the self-appointed 'cool' gang who squeal and pepper their conversations with "oh my god!" and "I was so like whatever" For them it's bonding tool, and a way of ruling their clique by excluding people.