One issue is perception of value. It's a lot easier to monetise a brand new game over one which is being updated or added to.

To everyone except hardcore gamers the current release method is probably very tiring, as in I might pick up a new game and play it for a few months at the rate I play games. In that time so many of these big titles would have come out.

It also means once these releases drop off best seller lists they quickly become unloved. I've seen big bugs and game mechanics issue that just never get fixed I guess because most of the sales have already happened.