Yeah but with Uber, the expectation isn't that you're going to leave a nuanced review about the comfort of the car seat and the smoothness of the ride, but to basically leave 5 stars so long as there was nothing horribly wrong. Which is fine. It's more or less a commodity service.
With restaurants, on the other hand, there is a wide range of quality. Even if the average shopping mall restaurant delivers a hot meal in a reasonable timeframe, I'm probably not going to give it 5 stars.
If reports are to be believed, then clearly Uber expected that it would be 5 stars or nothing, and their low tolerance for 4 stars makes reality more consistent with that belief. But as a user from the early days, nothing in the app clued me into this expectation and accordingly I gave 3 stars for "meets expectations", 4 stars when I was offered refreshments and USB connections, 5 stars when I couldn't imagine a better ride...
It's only lately and due to the prevailing rumor of Uber's mercilessness that I give 5 stars to meet "don't fire the driver on my account" and less than 5 stars to mean "I demand a refund and the head of the driver."
Back in the day when it was primarily an "Internet flea market", although it didn't use a star system, eBay was similar. Both sellers and buyers expected positive feedback accompanied with a comment like "A++++++++++ turned water into wine" so long as the reality wasn't "sent me a box full of bricks and axe murdered my extended family." Neutrals for a transaction that didn't quite go right even if issues were resolved were NOT appreciated.
With restaurants, on the other hand, there is a wide range of quality. Even if the average shopping mall restaurant delivers a hot meal in a reasonable timeframe, I'm probably not going to give it 5 stars.