Yes, the ideal situation might be a young family living with an elderly adult.
The problem is, people get attached to their "things," including houses. By the time people get elderly, they often have a paid-off house full of the fruits of a lifetime of labor. The logical thing to do would be to sell the house, estate sell the contents, and be someone's roommate using an online service like silver nest. The reality is people cling to their stuff.
It's not just the elderly who cling to stuff. How many 20-30 somethings are OK being someone's roomie, owning virtually nothing of their own, etc?
People at both ends of the spectrum want lots of space and lots of stuff. This ends in poverty for the young, and broken hips for the old.