But... he is self employed, which means a significant part of his work includes sales and management. That's something I don't enjoy doing and I am very happy having a boss do it for me. He also has more tax and less employee protection and other advantages, which he compensates by having insurance, but that extra expense is to be considered when comparing income (it is still more than me in the end).

As I said, I am content with my life, even though I know I could make is "better" if I really wanted to. It is just an insight on a reason why many engineers prefer to stay despite the "low" pay.

Also worth noting that many French people dislike the very idea of working in the US. Sometimes because they don't like the culture or the sometimes misguided idea they have of it. Sometimes it is the language, or just moving and leaving their relatives behind. Canada, especially Quebec can be a "milder" alternative, I have several friends who went there, some stayed, others came back, I think the pay was higher there every time.