- Working hours. If you are on the West Coast, good luck finding someone in Europe who will be available at 3pm your time.
- Superiority complex. I get the impression a lot of Americans treat everywhere else like it is a third world country. I work for a US company that has a branch in a post-Soviet EU country, and when the C-levels were discussing expanding our team, they were talking as if there would be a queue of people waiting at the door to interview.
Taxes and employment laws are really not an issue, there are plenty of ways around it, especially if you are a small startup. As others have pointed out, the simplest way is to treat someone as a 1099 independent contractor. You pay them gross, and let them deal with all their own tax responsibilities. If you want proper employees there are agencies everywhere who will deal with this for you, and in a lot of cases government agencies will help you and / or give you tax breaks as they want you to invest in their country.
(Source: As a European, I've worked half my career remotely for US companies)