I think it has to do with wages. And also with the ability to abuse those employees without exposing the "brand" to public scrutiny and litigation. It also allows you to offer better benefits to your core employees without offering the same to employees that work for contractors (there are fringe benefits like childcare or use of company facilities like private gyms that generally must be offered to all employees in order to expense them).
Oh, you all weren't paid for your overtime? Sorry, you don't work for Hilton. You work for Rent-a-Janitor. And they just filed for bankruptcy and closed shop. Hilton had previously dropped their contract with them due to their unscrupulous practices. We have now contracted with a totally different company, Rent-a-Janitor 2.
(This is a made up example. I have no idea if Hilton does anything like this, but most all huge companies do it.)
It doesn't, you're literally paying a middleman to take away the risk and hassle of hiring an employee. That's the added value here.
Otherwise, you could just hire the worker directly for what that middleman is paying them. Working as a contractor, you get a pretty good idea of what your "rights" are worth in pure dollar terms.
Oh, you all weren't paid for your overtime? Sorry, you don't work for Hilton. You work for Rent-a-Janitor. And they just filed for bankruptcy and closed shop. Hilton had previously dropped their contract with them due to their unscrupulous practices. We have now contracted with a totally different company, Rent-a-Janitor 2.
(This is a made up example. I have no idea if Hilton does anything like this, but most all huge companies do it.)