There was an interview several months ago in the one of the game magazines (OXM or EGM - I can't recall) where a developer was asked this very question regarding next gen consoles.
The answer: because consumers want better graphics.
Apparently, publishers and developers have conducted research which shows that better graphics are more important to the average gamer than solid framerate. Also, games with so-so graphics but stable framerates consistently get lower review scores than games with better graphics and less stable framerates.
There you have it.
This developer also said that in most cases they could improve the framerate, but they usually run out of time so you end up with great visuals and less than great framerates. This is just how developers have to assign their priorities. Graphics are higher priority than framerate.