The belief was that home owners build equity in their homes by making regular payments that include both interest and principal. For most families, paying a mortgage is a forced form of savings. But this assumes home owners have the cash flow that allows them to build equity in their houses. Encourage those unaccustomed or unable to save to become home owners, and they are apt to refinance and take any growing equity out of their house to fund other expenses. In fact, that is exactly what happened.
The idea was that purchasing a home is an investment. But the home you own is not an investment. An investment pays you money. Rental property is an investment. The house you live in is a liability, which increases proportionately with its size.