I think they're shoehorned(is that the right term?) into the gameplay. Kinda like they wanted to do a trilogy, but once you've done the first two you really don't want to change anything just finish it and move on.
I'd love the idea of a dino romp or hunting game in the engine, seems like there is a lot more you can do with the engine just gotta go outside the box.
Munching On: Archeage, SAO Vita, Tales of Xallia 1/2.