Sure but getting 20 people to live together for an extended period of time is logistically difficult. Two people who already want to be/live together can more easily commit to long economic partnerships like buying a house. It's important to note that while people have been getting married for a long time, having two incomes is relatively new and only recently the norm since women were discouraged from entering the workforce until WW2 and up until the 70s.

Some argue there is a "Two-Income Trap" where having two incomes did make certain things more affordable for the middle class like housing but eventually it simply drove up the price of housing & other goods. So now a dual income is simply normal and only having a single income is a disadvantage. Therefore it's not super surprising to see single young adults moving back home.

In certain super high CoL areas like the Bay Area we do see a bit of a trend towards 3+ communal living (post college) in the form of group homes. While this decreases the burden of real estate prices, in most cases residents are not building up equity in the home since they rent. In some cases they may be saving enough to offset the lack of equity but that's probably not very common.

Single income families are a very recent aberration that covers a tiny slice of human history. In medieval Europe for example everyone gathered the harvest.

It’s largely an outgrowth of extreme prosperity and urbanization.

yes its called a company