It’s mainly bc when they keep multiplying. As they keep multiplying, the RNA also changes. They sort of multiply similarly to mitosis. So, since viruses need a cell to be able to create more of itself, then it forces its RNA to the nucleus to make the cell only make copies of the virus. What would then make a virus evolve would be any changes in the RNA sequence. Like I said, this is similar to mitosis. In mitosis, the DNA sequence does change when it keeps growing and changing. With each person, their DNA is always unique and difference. Meaning that with a virus forcing a cell to make more copies, there might be a change in the RNA making the virus have a chance to evolve.