I believe they are also 1099 contract workers as opposed to having W2 employee classification. 1099 means you owe self employment taxes, which can take a significant cut out of your gross earnings (ball park 10-15%). You also may have to make quarterly estimated tax payments or you can get a penalty fee when you file in April.

Looks like https://ttlc.intuit.com/questions/2561815-uber-driver-questi... shows some of the tax issues related to being an Uber driver.