Well first of all because all those niches are already full beyond capacity. All the engineering, arts, cancer research, you name it, that the society can pay for is stuffed to bursting point and in no lack of pipelines of fresh wannabe recruits. Also cancer research is a deeply unprofitable enterprise overall, much in contrast to a domain like finance. One can labor a lifetime and get nothig, at least in finance they'll shovel some money from one pocket to the other and pocket the commission themselves.
Please leave people alone and stop suggesting alternate careers, that you yourselves did not choose. That's the uttermost hypocrisy. I don't wanna see anymore programmers with fat paychecks jumping on whatever latest fad like flies on a fresh laid turd doing AI and crypto and advertising and just bullshit pretend work at FAANG shedding crocodile tears on how the finance guys should be poor and suffering. Quit your high paying jobs yourselfes and go starve while doing biology PhDs then work as baristas while competing with the other 10,000 highly skilled Phds hoping to catch one of the 10 currently open positions in the world that pay a miserable salary with no stability in the future that does cancer research.
People imagine it's like prestigious professors at top research institutions leaving their posts because of the allure of money, when it's more like the 100x people just like them who were denied tenure and realized they chased a career in science like an inner city youth chases getting into the NBA: most people fail. Then what?
I even know people who worked in quant finance, struck it rich, quit to finish becoming doctors, and then came screaming back because of how horrible medicine was.