Originally settled by alchemists who tried to turn the sand into gold.

Or maybe they're people who used to live on Kamino and got sick of all the rain.

Or Tatooine is halfway between Naboo and ... maybe Alderaan, and back before the hyperdrive was invented they needed a pit stop. And the people who ran out of fuel or couldn't fix their ships ended up settling there.

Or maybe there's a secret underground cave system that's like a tropical paradise, and only a select few know about it, they never tell new arrivals until they've been around a few years. I think there was an entrance to the cave system in Ben Kenobi's hut.

:P