First off, the ones you propose that “sound reasonable”, in fact, do not sound reasonable at all. If the Chinese government were making a biological weapon, they would certainly be doing so in a lab dedicated to that purpose and would definitely have safeguards against it being released to the general public. While China is not the most modern country on the planet, they do have some of the most modern things here. And although their government is certainly corrupt, here’s a maxim I think I may have coined (sadly, I probably didn’t): corrupt does not mean incompetent.
Your example about the cancer cure definitely is the opposite of “sounds reasonable”. That one depends on the drug companies being more interested in profit. If that were the case, they would not be working on any other cure for any other disease and certainly would not be working on various vaccines.
The one about Oswald does not pass the smell test either. That would require a number of things to already be true, none of which happened or were even possible.
Let’s face it, those who believe in conspiracy theories do so because they want those CTs to be true. And why do they want them to be true? From what I’ve noticed from CT supporters I’ve seen both online and in real life (sadly), they are simply bigots, specifically anti-Semites with a side-order of white supremacy chucked in. They believe that nonsense because they want to blame someone, specifically Jews. They simply do not want to believe that something bad happened because an evil SOB did so all on his lonesome and thus some “powerful group” must be behind all the bad stuff in the world.
Of course this completely disregards the number of their target group who are not involved in the supposed benefit said group enjoys and it also completely ignores the reasons why their target group may seem to be heavily represented in certain fields. That last also requires some rather creative accounting. And by “creative accounting”, I mean “pure unadulterated ignorant horseshit and pulling numbers out of one’s tush”.
There is no coherence or internal logic to any CT other than prejudice. Look at the nonsense spread even today about Jane Fonda, Obama, Omar, Antifa, and BLM, to name just a few recent ones.
No, the reason the bigots buy into the “clearly outlandish” CTs is, again, because they want them to be true and the only “logic” they care about is their prejudice. Seriously, how many CTers have you met who are not also bigots? For me, that number is zero.