Individual landlords also dislike those and may not allow it, because the ability of the household to make rent now depends on all adults in that household. 3-4 broke adults who may only loosely know each other and with their own individual plans are a lot less stable than, for example, married couples. It's basically 3x the risk and hassle. Chances are you'd have to evict them after just a couple months.

They'd rather just leave the apartment empty and hope to find a better tenant.

> municipality starts banning adult co-living situations

Is that not typically happening only for more egregious situations? E.g. the ban is on more than 3 or 4 unrelated adults living together. There are plausible reasons why that should be regulated, it is not clearly a conspiracy by homeowners to prop up the value of their own homes.