Taxes don't really play on this --> many startups don't make a profit ;-) . Social Security might, but a talented engineer in France still costs less overall than the same one in the Bay Area and employee loyalty is higher.
One issue is uncertainty over government intervention, there is a bad history of the French government weighing-in on M&A activity. But the real issue is raising funds - while Seed rounds are generally covered by, among other things, government loans/subsidies, the ecosystem is lacking funds financing Series A and B. Here the government tries to provide incentives but pension funds are not the right level of financing for these rounds...the VC ecosystem needs to grow and that is a challenge because of a risk-averse culture.
If I were a VC in the US, I would see France as an opportunity.