I'm not going to claim it is an epidemic, but I know a few people in this boat. Generally speaking they are able to get by with a few life hacks: Cohabitate rather than marry, work jobs under the table, and/or be the stay-at-home parent.

Why would they allow their student loans to default in such a situation instead of enrolling in an income based repayment plan? If they have 0 income, they would have a payment of 0 dollars, still get their tax refund, and have their loans forgiven after a some time on the plan.

Not all federal loans allow income-based repayment. For instance, parent PLUS loans aren't eligible, nor are other loans made to or through parents. Moreover, there are tons & tons of private loans (often with high interest rates) pushed at these places, and none of those need to offer income-based repayment.

PLUS loans are to the parents though, not the students- do they even count as "student" loans? They are more "parent" loans.