First there is no 'one' cancer
There are cancers, subtypes of cancers, subsequent molecular variants and so on..
Therefore, there isn't a need for ONE cure, but a whole arsenal of options to be able to tackle them all. And many subtypes are just too nasty, they are a death sentence unfortunately.
There are endless teams led by principal investigators (PIs) who invest their entire careers reading loads of peer-reviewed scientific articles, formulating hypotheses, applying for research grants, hiring staff, conducting research, publishing their own peer-reviewed papers, presenting at conferences, forming international collaborations with other PIs to expand their output, and generally try their best to push the conversation forward in search of solution, potential molecular / therapeutic targets. There are then clinical trials, treatment protocols etc The fight against 'cancer(s)' is a concerted effort, moves slow due to the super complexity of the matter. There are novel drug discovery pipelines, some leveraging AI such as Isomorphic labs, that promise to accelerate treatment discovery. The next 20 years we might see things getting accelerated. But for beings who have been evolving biologically over a very very long time, millions of years, spending 30+ years for the discovery of a single cure of a certain subtype of cancer is seen as a short time. That might change soon, but the tasks is so ridiculously difficult we don't need silly conspiracy theories to make sense of it.
Ffs