In D&D, they serve as somewhere for summoned creatures and other alien monsters to come from. But to go to? Okay for a short visit, but not much point in staying there for any length of time.Hot take: Because most of us don't really care about planes.
Planes are like planets in pulp sci fi. They take one feature and exaggerate it to massive proportions. The "planet of" trope. Planet of trees, planet of water, planet of fire, planet of proud warriors, planet of pretty 3D VFX, planet of sand (I don't like sand).
So, once you have done the planet/plane of the week it's time to warp on to somewhere else.
I think players tend to like the comfortable familiarity of GenericFantasyland(TM) too.