It more or less started out that way in the U.S. Then we ran into a combo of reduced regulation (which all the pols who now want to point fingers of blame were heralding as a great advance in making homeownership more accessable at the time) and a rising market which many of the mortgage lenders had an interest in fueling because of the structure of their business.

In order to keep demand going (and prices rising), these loans were sold to some homeowners with lower and lower down payment requirements, since the down payment was a big obstacle to many potential buyers in a country with a near-zero to negative savings rate, going through "hybrid" loans where the borrower was loaned 20% as a consumer loan or even a HELOC and 80% as a traditional mortgage, then progressing into the Option-ARM loans (up to 100% financing), teaser loans, interest-only, and even negative amortization loans (where the sceduled payments didn't even cover the interest for a period of years).

Also, in some of the bubble markets, the vast majority of buyers were actually investors after a certian point. It was reported that in CA, option-ARMs and other exotic loans made up 60%+ of loans and around 90% of mortgages issued in the state in 2004 were to buyers who were purchasing a second home; there was no statistcal category for speculation in these lists, but I'd guess that almost all of those were looking to flip the houses they were buying since prices had been rising as much as 40% per year at the time, and some areas had tripled in price between 2001 and 2004.