My multisplit system costs <€60 a month to run even during the hottest months, which is way below heating costs during winter. And that's keeping entire apartment at constant 22ºC - people with higher "comfort temperature" can keep the bill significantly lower.

I got a 7000BTU one last week. One of cheapest I could find (I suspect some price gouging given the timing of my purchase). Very small, works in one small room. It was about £400. None of my family or friends have £400 sitting around that they can spend on a whim to save them a few weeks of suffering. A huge number of people live paycheque to paycheque and have no savings or credit lines available to them.

unless they also come w/ rooftop solar?

new mini splits are way more efficient than older systems as well.

insulation in older homes/buildings might be an issue though

Ironically- while often having solar on the roof.