You are taking account only of 1 of the gazillion taxes you have to pay in france, this one is only the employee part. Also the net amount in the employee pocket is way lower than this after medicare and unemployment mandatory taxes contributions (roughly 10 to 25% of salary). On his side, the employer have to pay in total something like 80% in additional taxes on the amount paid to the employee. That also explain why salaries are lower in France compared to other top 10 countries.

Source: until last year I had a company in France with several employees.