Psychologically, we associate digital games with technology at the most fundamental levels. Every game company--whether they know it or not--is half game developer and half R&D for the latest in computer technology, their twofold mission being both to deliver fun in a box and to find new ways to use the hardware they're given. Sometimes this just doesn't work for the better, such as in the case of Star Wars: The Force Unleashed--a game whose developers incessantly brag about cramming three middleware physics engines into it when they could've just faked two of them with a tiny bit of scripting here and there, or Sonic Adventure, a game which had the unfortunate distinction of both bringing Sonic into the 3D age and serving as a big tech demo for the Dreamcast. For those of you who don't follow that last example I'm referring to the multitude of playable characters and the unwelcome inclusion of a fishing game in the middle of a platformer.
The attitude that this tradition has given rise to is that each game should do something new. Raw technological innovations in gaming are becoming less impressive so more and more we're expecting innovations in interaction. The problem is that innovations like this, unless they're incredibly appealing at face value (motion controls), are really difficult to sell to the executives in charge of gaming these days, so innovation in interaction is an incredibly marginal thing. Personally I don't think developers try hard enough at it, but I do think that critics--Yahtzee in particular--come down a little bit too hard on them. Uncharted may be really similar to Gears of War and Tomb Raider, but it lacks the blatant sexism that's rampant in Tomb Raider and for my money represents a much more positive and appealing atmosphere than Gears of War. What's more, sometimes I'm just in the mood for a particular game but don't want to re-play the same levels, which is why I'm actually glad that franchises like Ratchet and Clank keep coming back.
It's got nothing to do with wanting to see stagnation or being overly attached to familiarity, it's just that sometimes people genuinely are just in the mood to play a little Mega Man every now and then the same way other people are in the mood to play basketball, cook, or build something every so often.