Speculating, but it could be a spiral where the shortage places more responsibilities on existing doctors, making the field less appealing to potential doctors. For too many people no amount of income compensates for a difficult lifestyle (we see this with air traffic control). If this were the case it would be productive to redesign medical systems to unburden doctors, but the current for-profit, scarcity-oriented system (in the US anyway) seems calibrated to squeeze everyone.
I have no idea what this might be.