Single biome worlds are in some cases justified. There really are planets where the climate and terrain is pretty much the same for most human purposes no matter where you are. Venus, for example, will kill you within 0.2 seconds no matter where you are. Anywhere on Ganymede is basically ice below and vacuum above.

And even if a planet does have a variety of terrain, in many cases a group of adventurers won't travel far enough to notice anything but what's in the local vicinity. It would be possible for Tatooine to have oceans and lakes and snow-capped peaks, and just for us never to have seen them because we never saw anything outside the desert region. Land in the middle of the Sahara and do a survey by foot and see what sort of impression you get of Earth.

So it's not entirely bad that all we ever see of Dagobah is a mucky swamp, or of Hoth a frozen field of ice.

Of course, canonically, these places are a desert world, a swamp world, and an ice world.

Transcript

GM: All right, I've spent all week designing... Utapau! A whole new planet!
Padmé: Cool!
Yoda: I've made a planet too! It's called Kashyyyk!
Padmé: Double cool!
Yoda: It's got three "y"s.
Padmé: Triple cool!
R2-D2: Whoa. Where did you get hex paper that big, Sally?
GM: I thought you were into fashion design.
Yoda: No, silly! Fashion is full of vacuous airheads. I'm going to be a fantasy writer.
R2-D2: Technically, if they were vacuous they wouldn't—
Yoda: Look, it's got a misty fjord made of granite pinnacles covered with forest, leading into a sheltered harbour protecting wetlands made of sand and giant mangrove-like trees.
Yoda: And further inland there's tropical rainforest, and deserts, and mountains, and there's snow and glaciers and big rivers.
R2-D2: Wow, varied terrain. That's way more realistic than any planet we've been to so far. No offence.
Padmé: Awesome. Now what's this Über-pow like?
{beat}
GM: Well... It's got sinkholes...