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.
Porque se eles soubessem o que estavam assinando, saberiam que assim que as taxas de juro começassem a subir, a prestação mensal subiria ainda mais, eles não poderiam mais pagar e ficariam sem casa.