Don't even get me started on Mustafar - love SW though

The way I treat legends is, it's true until Disney overwrites it.

It comes from the script.

Sci Fi tends to rely more heavily on the Fi than the Sci, and Star Wars is one elf short and one too many spaceships shy of being Fantasy.

Yeah, SW is definitely more "science fantasy" than science fiction. Good sci-fi doesn't defy known science.

Taking creative license to new heights,,,

It's space, nothing arboral has to be there for oxygen....there could be vast other sources of 02.....that's like asking why bother having moisture evaporators!

Oxygen is the most abundant element on the Earth, and the third most abundant element in the universe (after hydrogen and helium). However, being highly reactive, it is usually tied up with other elements in molecules. Photosynthesis splits it off H2O. No photosynthesis, no liberated oxygen in the atmosphere.

There would be plenty of oxygen on Tatooine as can be seen in that pic - in the form of sand AKA silica AKA silicon dioxide. But not in the air.

Caverns full of toxic algae.