It's not that they don't get it, it's that their past experiences with moneylenders always let them get out of the more annoying parts of said lending.
Because if they couldn't just use their titles and social rep for credit, they usually had the only real armed force in the area under their command. Each of the lower nobility was a king in their own right, and so they only paid debts to other nobles, since those were the only people who could collect debts.
Then they see this 'official' money lending institution, and assumed that it's just a nobility approved version of what they had before.
Only to find out 'oh shit, they were actually serious about those contracts'.