At least in the US there are more people who want to be doctors than positions available. The bottleneck is medical school acceptance rates. To practice medicine you need a medical license, which you can only get from an accredited university.

I knew students who were rejected from medical school, but I also knew far more students who were at one time pre-med, saw how much effort and debt that would-be doctors would need to take on, and saw the risk of pursuing an e.g. biology degree where their entire future hinges on getting into med school, and they chose a different field.