In many parts of Europe tax include social contributions, so you don't pay anything additionaly for haelthcare and retirement. You have free or almost free education up to the university level, subsidized child care, actual unemployment benefits, good transport infrastructure and many such things that you need to pay yourself in the USA. If you want to compare real effective tax rates, just add all these additional expenses to your tax amount and compare it to Europe.
Than again, the chart doesn't include US state tax, so you are really taking into account only a part of US tax rate.