The issue is that
1) One of the largest retailers, Wal-Mart, will not sell an AO rated game. So even if MS, Sony and Nintendo allowed AO games to be licensed, you'd sell like 10 copies.
2) But on the other hand, they will sell unrated (ie, no ESRB involvement whatsoever) PC games.
3) You have to try REALLY REALLY hard to even get an AO rating from the ESRB.
4) So if you're making a game that you know for sure is going to be AO from the start, you might as well just make it for PC/Mac and sell it online anyway.
However, I still do agree that a ratings system is kind of pointless when the "worst" rating isn't even usable.