Is that really such a surprise though? Think about it from a business point of view and not a artistic one. You make a game and it becomes wildly successful. Now you have millions of dollars to grow your brand even bigger so you can make bigger and better games. New young minds want to come work for your company with new skills and great ideas. Publishers are throwing money at you to join their parent company. Fans are expressing a desire to see something similar to what you just gave them. You know have the gaming world on baited breath.
Do you
A.) Make an entirely different game with no noticeable elements to the previous game, not even in a spiritual successor like way, but rather a completely different game in general. Like making From Software's Dark Souls, then following up with Bound by Flame?
or
B.) Make a game with 90% the same feel as the previous game even carrying the title with the appropriate sequel number, while only redesigning 10% of it for better game player, graphics, sound, and of course a entirely new story with new settings/characters?
I think companies should definitely make new IP''s every once and awhile, when a good idea comes along but trying to make a new simply because you want to stretch your legs or try something different when you have a winning formula is a bad idea in my opinion. Let the fans dictate when its time to move on, it's their money you want at the end of the day. So you build what they want, and when they get tired of that you go back to the creative pool and come up with something new.
Heck, just think about it from a fan's perspective - don't you want a game similar to one you'd enjoyed in the past? Final Fantasy has gone 15 games (not including spinoffs) of new characters, settings and worlds, using only the general principles of the past games as a source if continuity. And millions of fans keep asking for (and buying) sequels.
I don't begrudge a developer making an IP with its eye towards future installments/content, nor do I think it inherently taints their artistic slant. Longevity of a series is its own value to some gamers.