There is a simple way to know that we could train more doctors - failure rates are very low, which means that either the MCAT is a better filter than the exams (unlikely!), or that it would be possible to train a lot more doctors.
For med school specifically, you’re only taking the top performers, then of those you’re filtering further based on MCAT scores.
If you’re that selective and you already have a 15-20% failure rate, I’m not sure that lowering the entry requirements is a great use of resources.
You don’t really want a 40% failure rate. That’s a lot of wasted time, effort, and money.
There are already many alternate career paths for medical practitioners who can’t make it into med school or just don’t want to spend as much time in training.
That's enough for a minimum baseline of quality. Medical school in general is hellish enough to drive people to actual suicide and there's plenty that can be said about that but at least stupid people can't get into these schools.
It could always be worse. It could be a pointless system that inflicted hell on students and minted shitty doctors!
You should look into what's going on here in Brazil if you believe otherwise. Politicians here went wild with your "just open schools and train a huge number of doctors" idea. Gotta care for the poor and all that. The result is any moron can become a doctor these days. It was and still is an absolute horror show and it keeps getting worse. People are actually dying and I'm putting it mildly.
I hesitate to go into too much detail because the barbarity I've witnessed is just shameful. Let's just say that literally right now there are doctors working ERs who are not smart enough to diagnose and treat a myocardial infarction correctly.