The evolutionary goal for a virus is to have more copies of itself. Killing a patient 3 days after infecting them is a poor strategy for this, so the only option for these kinds of super deadly viruses is either kill a population outright and die out with the population (dead people don't spread viruses), OR if the virus is lucky it will mutate into a less deadly variant to be able to spread to more people.
The specific subset of viruses that will evolve to become less deadly have two properties:
1) They kill quickly
2) They kill most people
The problem with Covid is that it doesn't have these properties. It's only deadly for approximately 1% of the population, so it will continue spreading even if it ends up killing the host, because for every 100 people only 1 has stopped spreading.
The other problem is that Covid doesn't kill quickly, if I recall correctly, most people die after week 2-3. Covid will spread even before the host is symptomatic, so again there is no evolutionary advantage to mutate away this trait.