biology 101
every living thing's purpose is to survive as long as possible. the virus that survives the longest is the virus that does not kill the host and even better if it's harmless enough to not provoke an immune response, so the natural evolution of all viruses is to eventually become as harmless as possible. think of how many types of symbiotic microbial life exists in our bodies, many of these started off as deadly pathogens.
a virus like ebola that has a high mortality rate can kill off a population before it has a chance to evolve into a milder variant so that process for it is very slow but for a virus like covid19 that already has a very low mortality rate, this evolutionary process is very fast as indicated by the very mild omicron variant (which is the 15th letter of the greek alphabet, which would imply that this is the 15th known variant in under 2 years)