There are currently attempts to sort of do that, but it isn't usually as simple as modifying the virus to infect cancer cells. I'll explain more about that in a second. First, as far as a challenge to doing this that you asked about, the big one is that cancer cells are very similar to your healthy cells. It's difficult to produce a virus that would reliably target cancer cells without also infecting at least some of your healthy cells somewhere in your body. Cancer cells are human cells, so it isn't the same as something that targets bacterial cells.

That said, one method in gene therapy studies nowadays is to use viruses as vectors to deliver genes to cancerous cells. Simple retroviruses, for instance, infect dividing cell populations. Since cancer cells are actively dividing, retroviruses will infect them. One method is to engineer a retrovirus with an enzyme that will convert a 'prodrug' to a compound which will kill the cell. The problem is, if you're going to genetically alter a virus and put it in someone, you want to have a failsafe in place to prevent it from replicating and spreading. So, these viruses are then engineered to be replication defective...meaning they'll inject their RNA into cells and integrate it into the host genome, but will lack the genes needed to commandeer the cell and create more replicating viral copies. These replication defective viruses are grown in what are called "packaging cells" which are themselves altered to contain the genes that the modified viruses lack. (This way, packaging cell lines create the altered virus, which is then incapable of replication in normal cells). There's always the risk that these viruses could regain replication competence through various means, so they're engineered in a way to minimize this risk, and screening for viral shedding and replication competence should be performed to make sure it isn't happening. There are other ways to approach this with viruses other than retroviruses but this comment is already long enough lol.

Point is, it's being worked on but is very complicated so will take time, and may never become the 'silver bullet' that you'd expect. (Fingers crossed tho!)