It’s true that California taxes are high. But it’s also true that France taxes are higher. And, to be fair, bring with them higher entitlements.
And I maintain that taxes are just not high enough to be a major obstacle to entrepreneurship in France - and yes I am aware of the dreaded "charges sociales". The reason we hear so much about them is because complaining about high taxes is easy and not controversial. Unlike the topic of employee protection, which is deeply polarizing.
And I very much doubt that availability of qualified hires is the obstacle either since the level of engineering Grandes Ecoles is quite good (I might be biased there)
So, I guess factors for why France is not doing that well re startups would be Employee Protection, Kafkaesque bureaucracy when dealing with creating the company and paying the said Charges Sociales and lack of capital.
Of course effective tax rate is much lower than marginal tax rate. Effective federal income tax would be 17.1%, effective California income tax would be 6.9%. This would make effective total tax 40.2% (it was 42.6% in 2017, before Trump's tax cuts).