I believe the big xbox live update (that gave the avatars) required a hard drive, so anyone who bought the the xbox 360 core had to buy a hard drive. Also the game can be chopped up, it's just data.
A dashboard update is a completely different barrel of fish to a game. MS' own guidelines do not allow forced installation.
And no, you can't split skyrim up into multiple discs. Consider how ridiculous it would have been in, say, Oblivion, to go up to dive rock and then have to swap discs every 5 seconds to load the distant land
ME2 is a hub-based game with entirely separated levels, totally different. And the 360 /does/ have internal memory, quite a significant amount in the newer models, but nowhere near enough to install entire games to.