The same as for any other car: road accidents, road maintenance and so on.
Most countries, in practice, actually subsidize car ownership by taking back in road tax less of what is actually spent on caring for externalities.
Add to that that, in most of Europe, population density is such that lots of people are actually able to live without a car. Buying a car if you live in a medieval city centre is 100% "for fun" and makes life awkward for everyone (parking and so on).
There is also an argument that many "luxury" goods are "positional", as in you get value from a big house or an expensive car, not just directly from providing shelter or transportation, but by signalling that you are better off than the people with the smaller, cheaper items.
Some economists argue that by taxing these goods, society as a whole spends less on them, but people still get the positional benefits, because the person with the biggest house still can look down on the person with the smaller house, even if the house is only 50% larger, rather than 100% larger. If those goods also have other externalities, like cars do and sprawling suburbs full of mcmansions, then you get a double benefit.
Some of it though is a remnant of a more "puritan" time.