I don't see the point in modifying a virus if they can't make a profit out of it. What would be the point of modifying the Zika virus so that it causes microcephaly if there's no vaccine or cure for the virus (disease goes away on its own actually) or cure for microcephaly?
Also, you mention that people in labs actually have cures for some virus, but if that were the case, why wouldn't they announce it? It would be very profitable, from a purely ''people buy the cure'' point of view and from a ''marketing'' point of view, considering it would give more recognition to the laboratory. They'd only lose money if they release them for free, but who says they have to do it for free?
The problem with microorganisms is that they can mutate pretty fast and evolve and gain resistance/methods to avoid death, therefore finding a cure is harder. The best example are influenza viruses, which have a new vaccine literally every year because of their mutations. But I agree that for example, in HIV, it is much more profitable to just keep selling the drug cocktail they usually give to HIV+ people than releasing a cure.
Well the thing with cancer is that it's not a disease, but a group of diseases which have in common to have a disregulated celular cycle. So you can't ''cure cancer'' because it is literally over 100 different diseases. What may be done is to cure a particular sub-type of cancer. For example, breast cancer has these sub-types (actually more if you grab a proper phatology book but i cba to scan it now)
https://en.wikipedia.org/wiki/List_of_breast_cancer_cell_lines