I never got this either. Residents, from my understanding, are paid poorly. Hospitals charge immense amounts of money. Surely they can squeeze a few residents in there without begging the government for money.
Residents are poorly paid, but residency programs are expensive to operate and expand. The non-profit and public sector teaching hospitals which operate most such programs are generally doing better than small rural hospitals but they're still in a precarious state. They take losses treating a lot of uninsured and Medicare/Medicaid patients.