We can and it's a very active area of research, called gene therapy. An early attempt gave a kid cancer, so people are still pretty cautious about it, but things are much improved now, and patients with a variety of diseases have been successfully treated. http://en.wikipedia.org/wiki/Gene_therapy
Another attempt caused a massive immune reaction. That said, the amount of furor over a few deaths in gene therapy trials seems to be irrational: far worse things happen in cancer treatment all the time.
I really wanted gene therapy to be available 20 years ago and even went to grad school to learn how to make it work. It was strongly deemphasized at my school in a preference for classic small-molecule protein inhibitor therapy.
Gene therapy is truly non-trivial. It took me several decades of studies in genomics and protein functional informatics to appreciate how subtle and complex the problem is. I'm hopeful that we can make technology that enables routine personalized gene therapy a common form of therapy, but it is truly not an easy problem. Causing cancer and immune response is only the first set of easily observed (fatal) side effects.
