Because the ratings system is based on a formula that is different for each position
Someone like dirk may have a lower overall rating because he doesn't have the rebounding abilities/defense most big man do, but in the game he plays exactly like he does in real life. His offensive stats are still elite and thats all that matters.
The formula allows for more individuality for each of the players
Its pretty obvious I don't understand why every year people complain about Dirk's rating when every year he shows why he is rated below the elite players of the league. Dude is a great offensive talent, one of the best I've ever seen but other than that what can he do?
*waits for someone to bring up his ring*