Because Sandler either plays a ridiculously giving and naive person with saint-like qualities (Wedding Singer, Deeds, waterboy) or he's an insensitive jerk with no direction in life (Billy Madison, Happy Gilmore).
Big Daddy being the one role that's right on the fence.
I wouldn't say the movies are the same, but that's pretty much Sandler's range in a nutshell. Which is fine, really.