Most likely because their is an abundance of them.
Do I mind them? No, because they are usually great.
But the 3D sections are also pretty great, and we should've gotten more of them.
Meanwhile in Sonic Unleashed, you have the Werehog, which means that half of the game has no 2D sections at all.
And then the Day Time stages, whichi have like idk a couple fun 2D sections that are 25 seconds at max.
They don't overstay their welcome in Unleashed like they do in Colors.
But in my opinion, it's even worse in Sonic Generations because Classic Sonic is already a thing, and their is no point in having them in the Modern Sonic stages. But I still don't mind very much, because they aren't unfun, just unnecessary.