Mutations aren't the main reason. Pretty much the entire population has been infected, making the virus less dangerous because we all have some degree of immunity. Unfortunately there is evidence that people who got vaccinated have less immunity than unvaccinated, due to immune system imprinting on the wrong strains. But even then they're still less susceptible than a naive person getting infected for the first time.

This is true of most highly contagious illnesses: you get infected first as a kid, when you're relatively resistant to disease, and those antibodies keep you reasonable healthy for the rest of your life. Even in old age, when otherwise illnesses could do you a lot more harm. In some cases we have effective vaccines, and can shortcut the process by ensuring kids are unharmed by their first infection.

I've been infected for sure at least once (tested covid+), and was probably infected again recently (exposed to confirmed covid+). The first time was a flu with a moderate fever and an awful headache. The second time barely even qualified as a cold.