Because if they knew what they were signing, they'd know that as soon as the interest rate started to rise, the monthly payment would also increase, they wouldn't be able to pay anymore and  they would end up without a house.
S'ils avaient su ce qu'ils signaient, ils auraient su que, dès que les taux d'intérêts allaient grimper, les paiements mensuels aussi, ils ne seraient plus capables de payer et ils se retrouveraient sans maison.