Some group of people are guaranteed to become brain surgeons.. the demand is there, the training is there, so it is a near certainty. In fact in the US at least, you can know very closely how many brain surgeons there can be per year. Who will fill those slots isn’t too kind to all takers. It is dedication, hard work, and luck (ie ovarian lottery).
Also the majority of those that “fail” at obtaining a neurosurgery training. If they were on the path to medical training will end up falling back to any number of high paying jobs in medicine or surgery.
This study found no one did terribly well at day trading. Random chance implies there will be some outlying outcomes, but that is a fundamentally different objective than having too many people for few slots.