Because income tax isn't the whole story. The French also pay social charges: ~8% on salary (and, unlike income tax, no relief on the first €9.700); ~15% on capital income (e.g. a house sale, investments etc.). Employer's also have to contribute to employee's social security (~13%). So, total tax revenue is closer to ~45% of gross salary (excluding TVA tax levied on purchases made with your net income). See:
http://www.cleiss.fr/docs/regimes/regime_france/an_a2.htmlLong story short: the level of care is fantastic but it costs the country a fortune and there's huge waste within the system (seen it first hand). Note too: most French also pay for private health insurance to augment the default level of state care (for things like dentistry work, optician etc.).
So it ain't cheap but if I had a serious condition there's no place in Europe I'd rather be to have it treated.