The exploration is just extremely flawed. As I said, there's nothing really to guide you. Modern open world games have waypoints for a reason. I know a lot of people hate them because they sorta linearize something that should be open, but the whole point is that it provides necessary information to progress. Even games that don't have a waypoint feature had some way to inform the player what they needed to do next. Perhaps it was a cutscene, or a journal entry, it's something the player can look at when they're lost and figure out what to do.
Mario 64 is a great example of this, even if there was no waypoints, you knew you were going in the right direction because every mission is titled accordingly.
Sonic Adventure? More like, Sonic Stumbles Around.
Sonic Adventure 2 has it's own fair share of issues. It's streamlined, removing a lot of the fat, but now you're left with extremely linear levels. Not that Adventure was the most free form Sonic game ever, but none of Sonic/Shadow's levels even come close to the openness of Speed Highway. You could maybe argue Final Rush, but that's just a bottomless pit with rails and platforms every now and then. Even Treasure Hunting stages were designed to be linear with how the radar tracked emerald pieces.
Adventure 1 is too "open" and Adventure 2 is too linear. If only Sonic 06 wasn't garbage. It sorta addresses both of their issues, but in the sloppiest way possible. Kingdom Valley is one of the most open 3D Sonic stages ever made and it's all downplayed by how horrid everything else is.