Mortgage Company A is feeling virtuous and really wants people who can't afford down payments to be able to afford houses too. So, they offer up $300k and simply take 15% of your paycheck for 30 years until they've collected $900k.

A 30 year fixed at 4.5% would cost you $547k total over the life of the loan. Mortgage Company A is instead making $900k where they used to make $547k.

The problem with these loans is that desperate people accept them. It's the person who can't save enough for a downpayment and is getting kicked out of their rental that will agree to such a usurious loan.

It's the reason why we have a maximum interest rate to begin with. Because without it, it's possible to find someone in a tight spot and make a ton of money off of their temporary misfortune.