Why is it so difficult to c..

We may be able to. There is a noval process known as RNAi.

Using RNAi in mice we have been able to destroy the genome of HIV and Hepatitus C viruses. This is amazing. Once we figured out a way to get it into a persons infected cells, we may well be able to cure a virus infection. It'll take many yearsn though, but watch this space.

The futures bright, the futures RNAi

Viruses are amazing things! They have the ability to mutate, or change their genetic makeup VERY quickly. So as soon as we get a chemical that can kill them, they may be able to mutate so that that chemical no longer works. Also, we have a lot of research to do so we can learn more about viruses. We haven't known about them that long (relatively speaking), so we need to research into new ways to fight them.

no viruses can change and adapt and mutate all the time

There is no cure at this point for viruses. You just need a bit of time to get over it. Your body will, over time, manufacture its own antibodies that will fight and defeat many viruses such as the flu or the common cold. Many viruses can be prevented by being vaccinated against them- such as polio, measles, chicken pox, mumps etc. We still haven't found vaccines for diseases such as aids, ebola and others.