Timezones, navigating two sets of employment laws, payroll complexity, cost of flying them in (even infrequently), language issues, recruitment (e.g. understanding foreign qualifications, advertising), and so on.

Typically if companies want remote workers abroad they avoid most of these by hiring an outsourcing company, letting the outsourcing company deal with the local stuff abroad, and all the US company has to do is send them a pile of money.

Hiring US remote worker doesn't solve all of these (e.g. two states in the US might have different employment laws) however it does solve at least half.