Whatever gave you that impression? Uber has strict rules about how drivers should behave with customers, or about the maintenance of their car. For example drivers are not allowed to be aggressive in traffic (like taxi drivers), they are not allowed to talk with the client unless the client shows interest in talking, they can't smoke in their car and they are not allowed to reject orders, otherwise they'll suffer a pay cut.

I've talked with multiple drivers that confirmed the above. I also once complained to Uber about a driver that called me to convince me to cancel my order and they replied that the driver was scolded. I have the response in my email to prove it.

As a disclaimer, I'm talking about Uber from Romania, not from the US, but it's the same story, drivers being hired as contractors.