I do. Employees are a cost and a liability. They have to be paid weekly/monthly instead of per gig. They would have to be paid even when there's no rides. They have to be paid for if they end up sick, unable to work, on pregnancy leave. They need to have payments in their retirement fund. In the UK's case, they need to pay for the NHS. They need to buy them a car, and pay for upkeep, maintenance, damages and fuel.
And they would fall under normal worker protection, so no, they wouldn't be fired if they missed any target even only once, there's procedures and legal requirements and such before formally firing someone.
So it's completely in Uber's interests to only do contractors, no obligations or extra costs except for whenever they do a ride, and I'm fairly sure the compensation per ride is much lower than is necessary to operate a business.
Where is the employee's benefits in this? Does Uber pay much more than they would earn working for a regular taxi company? I mean sure it's a job that people can do whenever they choose, but not doing it is a luxury. Assume most people need a 40 hour work week, or the income of one. Can't get that with Uber without working 80+ hours a week, and even then it depends on luck, supply, demand.