I remember finding out Americans build walls differently than Europe. The stories about finding insect nests in walls sound insane for me to this day.

Well, old buildings follow old building code or none at all. There are still a lot of 100 year old houses here that were built without insulation, and a void is gonna look damned attractive to wasps, bees, and smaller birds.

Interior finish work is expensive, doing it over to put in insulation is even moreso. And to add insult to injury, the low R value of walls is a data point used by siding installers, to try to talk you out of ripping the old stuff off before installing new. Each layer has an R value, dontchaknow...