Really the main issue in Northern Europe is that houses do not even have blinds. In Italy or Greece it's not too common to have AC either, but people live just fine by shutting the blinds during the day.
Compare Antwerpen[0] or Brussels[1] with Milan[2] or Crete[3] or Barcelona[4]
[0] https://www.google.com/maps/@51.219842,4.4006387,3a,75y,64.8...
[1] https://www.google.com/maps/@50.8398344,4.3562283,3a,75y,90....
[2] https://www.google.com/maps/@45.4558029,9.1837501,3a,75y,29....
[3] https://www.google.com/maps/@35.3266049,25.1255372,3a,75y,34...
[4] https://www.google.com/maps/@41.3880801,2.1358851,3a,75y,256...
Coming from France it seems weird to me, and obviously blinds would come very handy right now, but there are obviously a lot of houses with no blinds in Belgium, I can see them from where I stand.
Unless you're talking about curtains or other things that sit inside the house, which are basically useless against heat.