I think you're downplaying the complexity of cancers, viruses are also sort of a single thing, if you don't think too long about it, yet you will never find a single "cure for viruses"
This doesn't stop viruses from existing in the first place, though, only from reaching you. If you wanted to stop them from existing in the first place, well, you would indeed have a bit of a hard time. Controlling mutation could theoretically stop cancer from emerging in the first place, if we solve the somewhat less hard problem of altering the whole body (rather than altering the whole planet, as with viruses).