But you /can/ play against the AI instead of other players. Just have to be connected online. There are fake leagues & competitions against AI teams of various skills - essentially all the UT player vs player competitions mirrored against AI.
More actually, arent there like 10 AI cups and only 3 player vs player cups? But yeah, I was just pointing out you cant play "offline", AI play definitely works. Playing against other players gives more coins, though.