Honestly it's not that Sonic Adventure has bad graphics - remember the visuals and graphics of SA1 were widely praised as being some of the best graphics in video games up to that point - but it's that every single post-Dreamcast version of the game, so the GameCube, PC, PS3, and 360 versions of Sonic Adventure, all have a variety of graphical downgrades from the original Dreamcast version.
I mean you know it's telling when there are literally mods for the Steam version of the game to make it look like the Dreamcast release.