They seem to believe that scheduling a future ride goes against their entire business model. I completely disagree (see comment in link):
https://www.quora.com/Why-doesnt-Uber-allow-customers-to-pre...
https://www.quora.com/Why-doesnt-Uber-allow-customers-to-pre...
Really? That's a surprise. I use Gett mostly, and you certainly can schedule it — so I don't see why Uber couldn't implement it either.