For your last bit, the reason I assure that is because a cure for cancer would allow for them to charge significantly more over the long run than current treatments as the person would live longer and be better able to maintain their income and insurance (chemo is physically extremely rough on the body).

Novel cures are difficult. This is because the low hanging fruit are largely plucked. Bacteria are really only a threat when not caught or when they develop antibiotic resistance thanks to the efforts of mid 20th century research. Fungi that infect humans are rare and we have effective in body fungicides. Animal parasites are largely handled by antiparasitics and changes to infrastructure and hygiene. And plant infections are so rare idk if they’re a thing. This leaves viruses and nonpathogenic diseases.

Viruses are caught by antivirals sometimes but they’re a real pain as they don’t really have enough biology to attack via deluging a system with something to kill it without harming you. This means you sometimes get stuff that works like hiv retrovirals which were the results of decades of nonprofit research. Or you get vaccines.

This is actually why smallpox had a vaccine but the black death doesn’t. Black death is a bacteria and you can treat it with antibiotics. Tuberculosis and leprosy similarly just have a cure and while they may leave you with permanent damage, that’s damage they did before the cure. The great plagues of mankind were systemically destroyed or defanged last century.

And that leaves endogenic (originating from within) diseases. You have stuff like cancer, which might be easiest to think of as a yourself infection. Cancer is any one of a staggering number of genetic mutations causing anything from a pigmentation cell to a neuron getting a few switches flipped and now it reproduces without responding to self destruct signals and begins spreading. We’ve begun curing cancer by cancer, but it’s difficult and since there are so many cancers that are all different many of these cures are pretty niche medicines. That’s where curing really is these days.

Because the rest is crap like organ system issues. In such instances there’s not really a switch to flip or a thing to kill to make the problem go away forever. If you don’t produce insulin what you need is insulin you didn’t produce. Same goes for all sorts of other hormones and enzymes. And stuff like multiple sclerosis, well that’s something researchers are still trying to understand better for even diagnostic purposes.

When it comes to conspiracies ask yourself who the most honest person who might have to knowingly advance the conspiracy is, how many of that person is there, and why did they keep their mouth shut. Medical researchers would have to knowingly advance that, and a lot of them.