Actually they do lately, and for games of this kind graphics have become increasingly relevant, games like Crysis 2 benefit from offer better graphics and a different scheme of gameplay. After paying COD:BO I could notice that the pacing and some elements of it were padded because is all too confusing and extremely hard, I like difficulty but the lack of juxtaposition makes the game less enjoyable to play and more stressing to go through, I fear the same thing could happen to MW3(I'm also aware that BO and WaW were develop by Treyarch, but MW are no longer develop by Infinity ward, is Sledgehammer who's got that work now). COD4 and MW2 were good examples of how to make the campaign and its gameplay work, but MW2 did upgrade in a lot of areas which are necessary for a game to succeed (Graphics have been always a benchmark in COD games, but lately gameplay looks dated compared to games like Crysis 2 and of course BF3, not to mention OF: Red River among others).
Because is obviously the same thing and it cant be justified how such thing cant be re-released as DLC (remind you that this time it comes with the COD elite thing which makes it more ludicrous), I expect from a sequel some evolution, if they are going to release at the retail price at least make it substantially different or evolve in areas which need evolving (eliminating or reducing quick scoping (or aim-bot indexes) to make the game more balanced would be nice and this is yet to be seen).
And better writing because MW2 writing wasn't particularly brilliant, COD4 was and that was almost 5 years ago.