You're paying a percentage of your income, if you're making above a threshold. This is in comparison to debt which has interest that makes the debt increase over time until paid off - regardless of whether you are employed in a well paying job or even whether you're employed at all.

Yes, under both system money is being paid back to someone. But crucially, there's no interest with an ISA. And furthermore, no money is taken until people get a job that pays a certain amount.