I used to think this myself in the past, but my opinion has shifted over time.
If a surgeon needs to do X number of cases to become independently competent in a certain type of surgery and we want to graduate Y surgeons per year, then we need at least X * Y patients who require that kind of surgery every year.
At a certain point increasing Y requires you to decrease X and that's going to cut into surgeon quality.
Over time, I've come to appreciate that X * Y is often lower than I thought. There was a thread on reddit earlier this week about how open surgeries for things like gall bladder removal are increasingly rare nowadays, and most general surgeons who trained in the past 15 years don't feel comfortable doing them. So in the rare cases where an open approach is required they rely on their senior partners to step in. What happens when those senior partners retire?
Now some surgeries are important but not urgent, so you can maintain a low double digit number of hyperspecialists serving the entire country and fly patients over to them when needed. But for urgent surgeries where turnaround has to be in a matter of hours to days, you need a certain density of surgeons with the proper expertise across the country and that brings you back to the X * Y problem.