This isn't even remotely true. In France, the highest marginal rate kicks in at EUR 160k (approximately $170k). To get the same marginal tax rate in California, you would need to be making approximately $350k (approximately EUR 330k), or more than twice as much. (Note that in all cases, I am excluding payroll taxes like US SSI or French CSG).
(or very cheap) healthcare, cheap university education, and a strong social welfare net
The Europeans paying the same marginal rates as their extremely wealthy Californian counterparts generally do not qualify for most of this benefits of the strong social welfare net, and for the most part have chosen to have the bulk of their medical treatment in the U.S., paying out-of-pocket even though they could get the same treatment locally for free or very cheaply.