I also found that they seem to value the tradition and style (quality of food, quality of dinnerware, quality of decoration, etc) much more than the company.
Before marriage, we (guys) would get together often. After marriage, it promptly stopped. And the accounting began.
As I said in another comment, the causes of the decline are plenty, and nothing really dominates. I've given a few reasons here and elsewhere, but I'm sure there are many more. It's a death by a thousand cuts. The reasons I give here are due to a certain sub-clique. It won't apply to others' experiences.
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).