If the majority end up treating 5 as average, then you also have to do the same and go with how the majority ends up using the rating system, which in turn forces even more people to use that definition of ratings, and so the cycle continues.
If you hold 3/5 as your average when 5/5 is held as the average for most people, you're just hitting the driver with a "this was awful" effectively.
This rating scale allows for some form of categorisation of the levels of 'badness' of the drive (e.g. differentiating between dangerous driving and jerky driving, between rudeness and general discomfort), but doesn't allow 'above satisfactory' to be separated from good or excellent.
But perhaps that's what Uber is going for with the rating system; maybe their intent is that 5 is meant to be 'above satisfactory' and anything better than that should be a 5 + a tip? If that's not their intent then I have no clue why they haven't redone the rating system already.
Also, those categories are subjective as well. What's the difference between "bad" and "really bad"?
Mutual graded rating systems suck. That's all there is to it. Because no matter what, it turns into currency. "Yes/No" and then base a rating on that. Rotten Tomatoes had it right from the start.
But since everyone got in their heads that anything lower than a 5 is somehow an assault on the other party, 5s are the "average" rating and there is no way to distinguish between average and good. You only know that he was bad enough to get dinged .3 stars over the course of his history with the app.