They kind of do. (I am so sorry, not trying to be that guy).

Look at HIV. The original strain is horribly deadly, but the strains that have evolved within the last decade are much more tame. It’s because the virus that kills its host doesn’t get to spread - Zombie outbreaks excluded here.

The flu is the same way. New strains always emerge, but they are usually not fatal to most even without a vaccine.