You're ignoring the time cost (or assuming that other people don't care) -- if the ride takes 20 minutes by myself or 30 minutes by the time the car finds another passenger going my way, and then takes an earlier exit ramp to drop off that passenger, next time I'm not going to select the shared ride option.

That's the only thing that keeps me from using something like Uber Pool today -- I don't want to spend extra time picking up/dropping off another passenger.