Again, shooting from the hip a little here but I think theres a couple reasons for that.

First, no one seems to be immune in immunoglobulin tests to the virus infecting them. Everyone can get the virus, some folks are just asymptomatic. So it's not really immunity you're referring to. There isnt a world where some people cant get and dont transmit covid. Being asymptomatic and being immune are 2 very different things, and generally speaking, the level of community spread we are seeing means most everyone at least is not immune to breathing covid infected particles.

Second, the range of symptoms from heart problems to neurological receptor reactions like taste and smell and covid toe and fever and not being able to breathe etc are actually all spectrums. And people land all over these spectrums. What a vaccine of this type would do is make the symptoms of the virus far more predictable, in addition to the side effects of the vaccine being smaller and more predictable as well.