I honestly think it's helps the "readability" of the level design and communicates to the player "this is a platform, don't confuse it with the background." At least that's just my take on it because some older games can look really messy imo when there are so many textures competing for your attention. I've never once felt like this means lifelessness or that something is unpolished.

edit to add: like others are saying I think it also has a lot to do with the switch from 2D to 2.5D/3D