You would have to be a conspiracy nut to believe that and think that every researcher, company, scientist, and the supporting infrastructure in the industry is working together to keep cancer cures away from us... without any leaks, too.
There's plenty of money to be made from a "cure" (and as others have pointed out many a time, there's unlikely to be a single cure, as different cancers present and form in ways that make it extremely difficult to target even one type, let alone all). Especially if you are first - there's no question an individual or company could make much, much more from a cure than treatments which are expensive to develop and rapidly are phased out.
On top of that, you have to dehumanize everyone in the industry and believe that none of the people have family members who have fallen victim to cancer. Even if a few CEOs and upper management individuals were that far-gone morally, you'd have to be extremely deluded to believe everyone is.
And if you think just one or two companies or a handful of people are keeping the big secret cure in their pocket.... no, that doesn't make sense, either. The way markets work, every company and research team is paranoid enough that someone will beat them to market, and they'll lose out on fame and money.
Not to mention the status you'd have as the one who cured cancer. That's the dream goal of many people who aren't even interested in the money, or at least not as much as the glory. Do you not know anyone involved in scientific/academic research?
And
if you truly believe big cancer is holding back a cure, what kind of a asshole would you be for not going in and stealing the cure for the world?