Sonic Robo Blast 2 takes a stylistic approach. For one thing, this emulates the classic games. For another, it allows us to focus on the platforming rather than on decorating the world realistically. Not to mention that SRB2's palette doesn't really allow for realism.
Sonic Adventure 2's textures would probably not be very effectively adapted to SRB2. If I'm not mistaken, the last serious attempt to go the SA route with textures was before my time. In the meantime, I'm pretty sure everyone is well aware of sites like these, but prefer to use more classically inspired stuff or original concepts.