Because they create more runs than any other player in the league.
Well, that depends on which MVP school of thought you believe in
1. The #1 player in the league, period, regardless of his effect on his team (how Al sees it)
2. The player who actually is the most valuable to his team (how I see it)
Hence why I can't support a player, no matter how good, on a last place team getting MVP. An MVP, to me, takes a basement/average team and lifts them to playoff contender level. How much value can an MVP (in the true sense) have on a last place team? Does he take them from sub-basement level to basement/average level? Look at Alex Rodriguez. If he wasn't on the Rangers last year, would they have been saying "what's up?" to the Tigers in the contender-for-worst-record-of-all-time competition? Well, I can say one thing: He's not there anymore, and look where the Rangers are.
That's value, I tell you what.