This is part of a growing class of "house rich, cash poor" people.
They can't afford to move because the sale price of their existing (oversized) houses would not be enough to buy existing stock of smaller houses, in spite of the crazy market. The old houses they live in are increasingly in exurbs or even age-restricted communities that the kind of new family that might need a house that size can't be in for totally different reasons.
Plus they might want to stay in that community. Maybe that's where their friends or family are. Their doctors. All that kind of stuff. And it's not unreasonable for a person to want to keep living where they have a social network.
They also can't rent out rooms or ADUs because local zoning laws arbitrarily forbid it either directly or by enforcing things like minimum parking requirements that are not achievable. Which would be one great way to increase housing supply and let people stay where they are; turn extra space into more housing. But these boomer houses tend to be in the most restrictive type of suburbs that stifle the rights of the homeowners and prevents sustainable growth.
They increasingly don't have pensions because those disappeared in their lifetimes. Retirements funds got fucked by a variety of financial catastrophes in the intervening years, so they're increasingly relying on social security checks to pay for their (mandatory) car, big ass house expenses, and all that stuff. They're living well above their means and even if they realize it and want to make a change, the actual ability to do so is a massive problem.
The net result of this situation is even more tightness in the housing market. Even less real stock, since the ability to downsize is so lousy.
This thread has a sure lot of angry people and boomer hate in it. Which I get, but this is all part of the same housing problem with the same solutions -- more low-cost/smaller homes need to be built and fewer restrictive codes/zoning rules preventing common-sense housing. A lot of people want to develop the properties that people want to buy, but city policies are often the biggest obstacle to them -- that and lack of financial products to fund development thanks to the gradual snuffing out of local banks.