If each pays for their own share, not so expensive. If you host and invite the same number of people, it gets expensive if you get outside food, and a lot of work/time if you cook yourself. Both are more friction than a restaurant, and thus fewer people do it. In a circle of 50, there will be 10-30 who just don't have the energy/budget to host, and eventually the others despise them for never hosting. The steady state outcome is that those people neither host nor get invited.

In my family, not surprisingly it's the wife who wants to host dinners. And because of the (IMO) high standards, it means she's missing for days to over a week. Life pretty much goes on hold in the household during that time. She can't watch the kids, so I have to. We both work and have little time, which really means the kids get neglected - and it's measurable.

If they lowered their standards, this wouldn't be a problem. But they seem to value those standards more than the social mixing.

If hosting was "simple food, no decoration, not the tidiest home", it would happen a lot more.

(Of course, if you host at a restaurant, that's the most expensive).