Because the computer-generated images that symbolize said other planets are generally done with some shitty-shit stupid noise algorithm to generate the surface rather than anything decent (well, at least it's not uniform noise), whilst the ones for planet Earth just use existing map data for the Earth surface.

As it so happens I've been working on a game that has planets, so here's an example generated with better algorithms:

PS: also note that for game purposes, the athmosphere is unrealistically thick as a proportion of planetary radius, purelly because it looks better. A lot of choices in game making are mainly artistic freedom which at first people with a Science or Engineering background tend to shy away from "because it's not how things are".