I think in part because speculating what the land looks like on an alien planet is actually really hard to do, and the vast majority of artists just wing it. With sufficient planning and rigor, alien planets should look normal.

For instance, I think the landmass of Tira-292b looks pretty natural. It's a hypothetical planet created for the Alien Biospheres project, a YouTube series that tries to build up an alien ecosystem as accurately to science as reasonably possible

It's a seriously underrated series, I highly recommend everyone check it out