Why is it that a single person can do better remakes than the whole Nintendo company?
Compare the following Super Mario 64 remakes:
Nintendo
Fan
Such a remake would require basically remaking the entire game. At that point, why not make a new game instead? I'd rather have a new game than a HD remake of the exact same game, with the same old gameplay and dated level design.
Such a remake would be a huge undertaking, and Nintendo has limited resources to spend on developing new games, which means if they remade Super Mario 64 it would come at the cost of getting one less new game. Do you really want that?
I wouldn't mind just having some improved textures and models, and a new lighting/shading engine. That would go a long way towards making the game feel modern again, without a complete remake. It would still obviously be an old game, lacking the same degree of polish as newer games, but that's OK. I'm getting 3 games for the price of one though, so I can't complain.
If they were full remasters, rather than just ports, they'd be charging full price and selling them individually. I don't think it's a bad price for 3 ports, especially when 2 of them look great in HD.