I have no links handy, but practically every report by NHK and other outlets all come to the same conclusion that medical workers are leaving the industry and newcomers (medical students) aren't coming in because the pay is too low for the intense workload required compared to almost every other industry (and the shortage leads to doctors putting in even more hours, causing a negative feedback loop).

The low pay in turn has been linked to hospitals running on essentially zero margin, so they can't pay more even if they wanted to. That in turn has been linked to Japan's national healthcare insurance system and other government mandates, with arguments everyone here has probably heard enough already.