Well I didn't accidentally rate them a 3 - I picked a median score for standard service. I don't remember being told to do anything else when I signed up. If you rate 5 by default where do you go for truly special service?
There are a lot of ways in which a ride can be unsatisfactory (smelly, creepy, crappy car, poor driving, loud music, etc.) but not that many in which it can be exceptional. Unless they show up in a Tesla Roadster or something.
i.e. If I always rate 3, but sometimes rate 1; then that might be similar to someone always rating 5 but sometimes rating 1-3.
Ah, but then a user might throw an Internet Fit(tm) and cause a public outcry about ratings being manipulated by Uber etc. etc.