Activision knows that people will buy the same game (With small tweaks) every year. Don't see the problem with them doing so. :shrug: They apparently know how to make money. (And a lot of it at that)
I'm pretty sure I know why:
Person A pre-orders and tells all his CoD friends that he's moving to MW3
Friends of Person A pre-order too, because they have no choice if they want to continue playing together
Friends of those friends are told and they pre-order too so they won't be left out
etc.
So for every person who pre-orders, they cause a chain reaction of more pre-orders.