I haven't looked at this in awhile, and I know some legislation was just passed for more GME funding, but I believe the CMS funding for residents was capped ~15-20 years ago. So while some programs definitely get paid for their residents, I think basically all of the expansion has been self-funded by schools (there's a cap on funding from the government, NOT on residents).
Which, to me, is even more problematic. The days of altruistic academia are long gone. Perhaps there are some pockets left, but on average, the institutions are going to do things which make money, not lose it.
Knowing how my own residency program worked, talking to people from institutions of every size...I think they're far more similar than we realized. The resident workforce is leveraged to generate more RVUs for the institution, through no "grand scheme" but just human nature.
If you're an Assistant Professor at XYZ Health Systems, and your promotion to Associate Professor depends on grants and papers, but your salary depends on RVUs...what is invariably going to happen? If you have a resident, you're going to "allow" that resident to pick up all the work they can because you need "academic time". The resident doesn't know any better, they're "getting service for education". You justify your actions because, well, the system has also built you a mouse trap. No one is doing anything nefarious.
Zoom out, and an academic RadOnc department over the last 15 years has taken over community hospitals and named them "satellites", faculty can be pushed with higher RVU targets while still maintaining lofty tenure requirements because residents have expanded and can be leveraged to make everyone work harder. The residents are ignorant because there is NO WAY to have any life experience prior to this to stop and say "hey, wait, am I being exploited?"
Repeat. Repeat.