You make no mention of the need to sell. They are more likely to move areas early in their lives as they change jobs. Or up-size if they start a family. This becomes difficult in negative equity, as they cannot discharge the mortgage on the sale of their house. Some lenders will actually block sales. The young buyer then often ends up with a bigger debt.
Young people are not thinking of their home as an investment. Surely you remember the excitement of buying your first home? We had a big mortgage and would have never been able to set up our business, employing staff for 30 years if we had been crippled by debt in our 20’s.