Bobby Kotick and the rest of the senior executives at Activision make the most money by pumping out the easiest crap to make (read: same engine as used in previous titles with light graphical updates)in the shortest amount of time as possible. It's in their best interest to milk franchises to death because the second it no longer becomes profitable, they can all just walk away and leave a sucking crater of creative void, happy with their short term profits.
It's like this. Would you rather:
a) Receive $500,000 tomorrow, and a pat on the back from the board of investors, or:
b) Receive $100,000 every month, knowing you might get fired at some point in the next 6 months because your title(s) wasn't/weren't as successful as expected. (You would continue to receive installments of $100,000 after the 6-month post-release period)
Greedy bastards (and those that play things safe), would choose a. Those that have an actual creative investment in the product (or backbone) would choose b. Hence; Tony Hawk, Guitar Hero, and CoD.