You can't buy a window unit? I literally don't get this...explain how you can't go to a hardware store in your country and buy an air conditioner or order one online
A lot of houses were built before window units were a thing, so you get windows that just won't fit them, and you need slightly to moderately more complicated systems to make it work, like "air conditioner bolted to wall with a hose into the window or through a wall" style.
Their climate is also on average vaguely more forgiving in the heat, with temperature ranges that are comparable but lower sustained highs and generally lower humidity resulting in generally more tolerable conditions even during the warm season.
Remember that Europe is much further north than we typically think. Italy is as far north as new York, and Germany is about as far north as Canada. "Why aren't air conditioners as popular in nova Scotia as they are in Florida" has a more obvious ring to it.
Finally, if you're used to it it's not a problem you feel compelled to solve.
The northern most states in the USA also have the same arrangement. It's (historically) in a cooler climate, where a "heatwave" is anything above 80F, so just open your windows if it's stuffy indoors. Combine that with fossil-fuel heating, and heat-pumps just aren't a thing.
It's pretty much the same here. Fossil-fuel heating is fairly rare though.
Heating is usually done with geothermal heat-pumps, district heating, direct electric heating, or with regular heat-pumps which are actually fairly popular.
Most of my friends in Germany and UK do not have AC, or have such undersized units for their homes it barely makes a difference. Or they don't want to run it because energy costs to utilize it during the day are ridiculous.
What I've learned at least about the houses in the UK is that the homes are old and drafty so it's not about the size of the unit but the insulation being poor.
Shit insulation is actually more of a pro argument for AC. I mean, the houses in the USA are made out of cardboard and gipsum. They dont insulate a thing. My house has great insulation. In summer I just keep the windows closed and roll down the shutters during daytime and it's freezing cold inside. At night we get cool wind from either the North or Baltic sea, we then open up the windows to freshen the air up a bit.
North vs South. AC is not very common in homes in countries like Netherlands and Germany. Mostly because it’s only really hot for one or two months and those are the months that Germans and Dutch people are on holiday. So it’s either go on holiday or stay at home for one summer and buy AC.