having now replayed all of sonic adventure DX, i think sega hasn't remastered it because then everyone would realise how... well it's fun, but a lot of the fun comes from the jank and nostalgia.
the jank is also super annoying and if there was a modern version to fix all of it, then idk if it would be the great game that everyone makes it out to be. sega REALLY wanted to do the "let's turn sonic around sideways and shit!" concept that, feels horrible. (see sky deck & lost world snake)
also, every tails level is padding. it's normally 50% or less of a sonic level that you have a time limit to beat. the level choice is sometimes bizarre too, why casinopolis??? you could've had RED MOUNTAIN, that level has so much open area you could've used??