there are a few reasons; here's one:
you start the game, you play 1.e4. your opponent doesn't move. their time does not start ticking. you wait. you wait. thank goodness, you click abort!
or another:
you put out an open seek. then some guy accepts it. unfortunately, it's a guy you played once before who exhibited bad sportsmanship, and you don't like playing with ... guys like that. so, you click abort!
Why do we allow players to abort games? After accepting a challenge, this allows a player to view the opening move and abort if he doesn't like it. It seems to me that once you have accepted a game, you should be committed to it!