However, what isn't covered (and this is especially pertinent for ride sharing) is the time between ending a job and taking the next job.

While I wouldn't expect anything like full-time pay, Uber/Lyft et. al. still get a benefit of having drivers on call (lower wait times for their customers). I personally would have liked to see some compensation (within reason) for time spent on call but not on a ride. Not sure how easy it would be to make both a fair and reasonably un-cheatable system, but its something that should have been considered.

They essentially do this at times with their bonuses, for example to setup a new geo area or to ensure rides in a certain area.

However if you incentive being on call no matter what, you get drivers driving to the mountains to get their guaranteed pay.

This is correct. Although it's also true that drivers may have to spend quite a bit of time waiting for a job to become available, which doesn't count as engaged time.