Gemini says: Over the past 20 years, approximately 1.8 million to 1.9 million Europeans have emigrated permanently to the United States (measured by lawful permanent resident status/green cards), while an estimated 1.6 million to 2.0 million Americans have moved to Europe on long-term residence permits and visas

I'll just say that I don't think the reason is a lack of knowledge and leave it here.

UK doesn't count, becausd it suffers of many of the problems that affect the US. And especially here on HN, there are almost no US citizens that have moved to Europe, and many European citizens that moved to the US, and can talk out of experience.