I think a game has to pass the 11 minute mark to get full IP/Exp. I could be wrong. Also the reason is because from a strategy perspective the game is hardly over by 25 (I know some games are but lets think of most "balanced" games). All it takes sometimes is 1 good team fight, an ambush at Barons, or just some people having that mental awakening to turn things around.
On the flip side sometimes the dominate team gets too cocky, DCs, ect and they lose the game for themselves. Some champions bloom late and that also contributes to the overall strategy. I believe Riot stated their goal was that the average game start to finish should take around 30 minutes to complete which is where Riot (combined with player feedback) thought that 25 would be perfect.