Because that's pretty much all the character really has. It always has been. He's Spider-Man so he can't actually grow and change. He can't settle down, do something different. And as Spider-Man he can't veer away from being the 'with great power comes great responsibility cuz Uncle Ben died' Spider-Man.Why does the third game need to have someone for Peter to be attached to? I think there's a lot that could be done with Peter moving on from being Spider-man just for himself and because he wants to. And also, Mary Jane. I don't know what they did with her in 2 though.
More than that though, Spider-Man is a brand. The supposed passing of the torch to Miles is allowed only in so far as Peter always being ready to jump back in as Spider-Man at a moments notice. Any actual change to Spider-Man, whether it be the symbiote suit, settling down, or fully pursuing a life of science, will only last so long till he's back again as the regular old webslinger.