Why don't most people stay friends?

How come when most relationships end for some reason the guy and the girl never end up being friends at the end ??

because then the friendship would be awkward because the girl/guy would know the other likes him/her

Its not easy to go from lovers back to friends. Relationship breakups are rarely mutual. One person is usually the heartbreaker, while the other mulls over the breakup and pain for weeks, even months, but the person who does the breaking up feels like less of a bad guy by offering that sense of truce: It's not you, it's me. We can still be friends. This peace offering of friendship provides the dumper with the comfort of knowing they aren't such a horrible person because they still want to be friends with their ex.
An ideal would be for exes to succeed at being friends, but in one where bitterness, jealousy, passion, and human nature exceed reasoning and rational thought, it's impossible. So unless the two of you were the best of friends before; both broke up on the same terms in a perfectly mutual breakup; both have no qualms about either of you seeing new people, its usually better to leave the friendship behind... along with the memories.

im still friends with all my ex boyfriends.
2 of them are even my best friends .

im still friends w/ my exes.. but the reason most quit being friends is because its really awkward for both.. like for example. I have a best friend named shane he is one of my exes and also my first love or whatever but its still really awkward between us.. we arent as good of friends as we were before the relationship.