I know a group, where 8 people + a dungeon master meet weekly/bi-weekly to play DnD. It's a little on the larger side but this would be "dinner" party size. Of course they bring food and drinks as well. It would be a little ridiculous (and expensive) for them to rent a grand ballroom every weekend.
Similarly there are plenty of people I know who look forward to college football and hold similarly sized game days with themselves and significant others. Again nothing special, just 6-12 people ultimately watching tv. Food is brought, and drinks are had, but they aren't having a "rager". Renting a park every weekend to watch college football every weekend would be expensive.
The fact that you would think to rent a park for these kind of things just leads me to believe we are talking past each other. We are missing significant cultural context on the utilities of these rooms. If you don't watch sports or don't play DnD I can see how the idea of needing a dining room is superfluous.
And most people would't throw away their friend groups in order to invest $10k on the S&P. At that point, why even have the money?
I do have a couple rooms that can accommodate 8 or so people each and I use them when I have guests. Also outdoor space in the summer. But I use those spaces for other things as well.
I have s living room with kitchen which is 24 square meters, definitely not big, contains the kitchen, sofas, tables and I hosted 10 people in it.