a virus is a virus, they all have the same effects on a body, and the epidemiology is the same as any virus
Different viruses have very different effects on a body. In fact, the same strain of viruses can have different effects on two different bodies.
Humans have immunity to Coronaviruses in the range of 6 months to 2 years. No one knows how long we have immunity to Covid-19, but it could easily be longer or shorter than that. That might be the reason we are nowhere near herd immunity.
Or maybe there is another reason. Whatever the reason, herd immunity has not panned out so far. Hoping for the easy solution, while refusing to work hard, has yet again failed.