In some cases, they can. A friend of mine is a project manager for CC Bill, a payment processor for porn sites. They hire almost exclusively from eastern Europe for software development. But if you want to do business with the US government or financial or healthcare industries, i.e. where most of the money is if you're not an ad tech giant, you're largely restricted to hiring US persons. That doesn't mean they need to be citizens, but they need to live in the US, and live here legally, which largely means they can command US wages.