Just because they tend to move towards less lethal doesn’t mean they never can go the other way, as evolution is not a plan or choice. Something increases lethality but extends overall illness time, increases transmissibility, increases asymptomatic time, or a combination would outcompete other less lethal versions. While, in the long run, a less lethal virus would be more beneficial to the virus, that wouldn’t stop a more lethal version from burning through a population until it dies out by destroying its own habitat.