As someone who runs an incubator in Paris but lives in Silicon Valley, let me add my 2cents:
look at what cost an employee in France
An engineer in France is way cheaper than an engineer in Silicon Valley. Taxes on salary are indeed higher, but the base salary is so much lower that in the end, France is cheaper. To give you an idea, a basic salary in Silicon Valley is $100K, the equivalent in France would be below 50K€. Those cost the employer about $120K and 75K€ respectively, so you can see which is cheaper.
On top of that, France has a pretty good tax incentive for R&D engineers. You can almost get all your money back as the employer, it's frankly crazy (it's called CIR). But nice for startups.
The main issue in France is the complexity of navigating all the government aid. It's a patchwork of measures that don't fit well with one another...