Carpooling has fundamental challenges that are perhaps impossible to overcome. For one, dynamic routing for two or more people is always going to hurt the user experience. Imagine you're running late and your car re-routes to pick up another passenger - ugh! Shared transportation for anything more than a neighbourhood trip should be professionally driven and to a schedule. That schedule can be dynamic as anything but…
In Germany, carpooling has been a mainstream thing since the mid-90ies or perhaps even earlier. The majority of trips are long-distance, i.e across-country. You are right that carpooling has has some shortcomings but it's so ridiculously cheap that many people happily accept those shortcomings. I used it many times when I was a student and 90% of the trips went as planned.