I don't think it's quite as prevalent as you think, it's just that it's the BIG games that involve saving the (area of space spanning at least half a planet). In fact, I think you'll find some to most terrific sleeper hits involve saving people and stuff on a much smaller scale.
The best example I can think of off-hand would be 90% of the entire Mario franchise. Save the princess save the princess beat Bowser beat WART etcetera etcetera etcetera. Except for the various Mario RPG games, the fact is you're going through a lot for... well, not a lot, I guess. Hell, in Super Mario Sunshine, you're fighting to clear your name.... and then you save Peach, but still, it starts as glorified community service!
Oh, and let's not forget all the games that have very little to do with saving stuff: Assassin's Creed, God of War, all racing/sports games, party games, and so on and so forth.