If any of you know, please ReBlog an answer. The Fremen created their own problem. They're sequestering vast quantities of water, removing it from the planet's atmospheric cycle and hiding it underground. If you've removed millions and millions of gallons of water from your environment, then of course you're going to live on a desert planet. Their great plan to transform the face of the planet would not be needed if they hadn't overreacted to what was probably a moderate drought a long time in the past.
Spice is what makes space travel between planets possible.
Spice is only available on Arrakis. I expect that a prequel book will show that the thinking machines that were destroyed in the Butlerian Jihad may have made space travel possible, but that's really the only answer I can think of that makes sense. That or there were people in the past who had the natural talent to create tesaracts without the aid of spice . That could be a lot of fun. I think I'll go get something to eat, then fill a few replacement part requests.