There are a lot of bizarre path dependence issues and misaligned financial incentives in the US healthcare system. Most teaching hospitals are non-profits, often run by university systems or governments. Those hospitals get much of their revenue from Medicare/Medicaid. Reimbursement rates are fixed and hospitals have no ability to raise prices to cover increased training expenses. So, the only solution has been to get separate GME funding from Medicare.

There are other GME funding sources such as private charitable foundations but still the majority of the money comes from Medicare.