I think you're ignoring that hiring someone onto your team takes a long time and significant effort. Say you're allowed to add one person to your team. You interview 50 people and decide on the best candidate. Then you spend the next 6-months waiting on lawyers, processing visas, moving their them and their family to the US, paying for their initial housing, etc. Might not be an issue for a small company in France that hires locals but it sure is for big companies in the US.