Because if a middle class American sells their middle class home and buys a European middle class home, they have an extra half million left over. And if a middle class European sells their middle class home and tries to buy an American middle class home, they're half a million short.
Neither the US nor Europe has such a unified housing market that this could be a reliable statement. In Europe it differs greatly within countries, and there are many countries. In the US it also varies greatly within each state and there are many states.
Also at this point both have seen such negative upward trends in housing prices that the assumption that someone placed in the middle class owns a home at all (without any debt attached) is questionable. (But that's just a comment and besides the point)
That's why people compare spending power instead of how much money you earn, because that comparison is meaningless.