A relevant factor is that the US has both a much higher income and a vastly lower savings rate than in Europe. Also, much of the wealth in Europe comes from hereditary property, which is not really a thing in the US. As a baseline, median Americans have much higher discretionary income than in Europe, so it is only savings habits that prevent many of them from accumulating wealth.

Americans with savings rates that are comparable to Europeans end up accumulating tremendous wealth over their careers, even in the middle class. This is offset by the majority that spend every penny. That Americans don’t save their prodigious income is an unfortunate artifact of having multiple generations of nearly unbroken prosperity.