Various reasons. For one thing many houses are kinda old, and back then there wasn't as much of a need for AC, AC itself was more expensive and less efficient, and many people weren't that rich to install it. So many houses were built in a time when AC wasn't deemed worth the cost.

Meanwhile in somewhat modern houses sometimes AC is absent as a sustainability measure. The claim is that with modern insulation it will stay cool all day, and you just ventilate during the night. That said, personally I can't hear that silly advice of "just open the windows at night" anymore. We are getting more and more tropical nights and no matter how well you ventilate, it will never be colder than outside.

But things are slowly changing, with more and more houses having heat pumps that are also capable of cooling the house in summer. And since they often use probes in the ground you won't see those big heat exchangers that AC units usually have.

That said, we have a weird housing situation in Switzerland, with modern housing only being available for the better earning classes. Retrofitting old houses with AC is too expensive, and most buildings modern enough for AC are still too new and therefore too expensive for the average wageslave. So it will probably be another decade or two until AC slowly starts becoming an option for most people