So you think that complicated diseases are easily curable and the entire scientific world, including very different countries like China, has just decided to hide the knowledge?
If your cynical take was correct, there would be no cures ever. And yet there are new ones all the time. For example, vaccines. There are way, way more vaccines developed in the 21st century than in the 250 years before that.
Vaccines against HPV have reduced incidence of cervical cancers to basically 0 in the cohorts that obtained them. How come? Shouldn't Big Cancer be interested in treating cervical cancers expensively and promoting relapses?
Even in cancers, your chances of surviving, say, Hodgkin's lymphoma, are now north of 90 per cent. The treatment is expensive, but time limited. You don't have to take pills for your entire life.
How does that square with your view of the medical system as a machine for prolonging diseases indefinitely?
Plus even if we posit nefarious forces, we should also account for nefarious forces which want the sickness gone.
If you're seriously sick you aren't making money because you can't work or all your money goes to Evil Pharma Co, then the Evil Government doesn't like that, because they can't wring taxes out of you. (Which they prefer since it's easier than fighting Evil Pharma Co.)
Meanwhile, The Shadow Government wants you to be healthy enough to work every day, or else they won't finish the navigation beacons for the alien invasion.