Under IBR, PAYE, and REPAYE, you pay anywhere between 10-15% of your adjusted gross income monthly for 20-25 years, depending on the plan. Lots of people are on these plans because their loan principals are too high to service with their income. As a result, the 10-15% they're paying typically doesn't do enough to pay down the debt meaningfully, resulting in interest piling up and the total debt amount expanding over 20-25 years.
The US tax code counts discharged debt as taxable income, no different than a wage you're paid for labor. Current tax law does NOT provide an exemption for debt discharged under IBR, PAYE, or REPAYE (only under PSLF, the ten-year government/not-for-profit plan, a much smaller portion of those under income-driven plans), meaning any debt discharged under those plans after 20-25 years will count as taxable income to the extent of the debtor's solvency.
Needless to say, this will be financially catastrophic for individuals who had to rely on an income-based plan to service a large debt in the first place.