There’s a little more nuance to it. The Postal Accountability and Enhancement Act of 2006 [1] is the major contributor to USPS’ ongoing financial issues. That act requires USPS to maintain a 75 year reserve for retirement and medical benefits. Other agencies including the Armed Forces max out at 20 years by comparison.

1. https://www.congress.gov/bill/109th-congress/house-bill/6407