In the US there've historically been great work and wealth-generating opportunities that weren't as readily available in Europe. That seems to come at the price of less safety net if something goes wrong e:g health problems, disability, job loss. In recent times Europe has become more like the US in the sense of cutting safety nets while being more entrepreneurial. I think this'll lead to less people choosing to move to the US from Europe, compounded by US now having possibly less opportunities and an administration that makes even well qualified legal immigrants feel unsafe. Which will become self-fulfilling, the opportunities of the future will increasingly be outside the US. As to why more Americans haven't historically moved to Europe, my guess would be its simply unawareness of how actually for a lot of people it'd give a better quality of life.