COVID isn't particularly deadly. The selection pressure that can cause a virus to become weaker is mediated by the virus dying off in the corpses it creates. People aren't dropping like flies from COVID, so that selection pressure isn't there, and the virus is free to mutate into more virulent forms. There's no reason to expect a relatively mild, yet still deadly, virus to become weaker, especially if our medical infrastructure keeps people alive that would've otherwise died from infection.

Virus evolution tends to optimize for replication and transmission, and not reduced severity. If a particular mutation causes a virus to replicate better than other variants, but it ends up killing hosts more often, it doesn't really matter what happens to the host afterwards, evoluntionarily, as the virus has already spread it genes more than it did before it mutated.