A good question. The following article talks about natural immunity;
https://www.zerohedge.com/covid-19/why-there-such-reluctance-discuss-natural-immunity
[But that long discussion does not answer the question. And it is an easy question to answer. No one wants to talk natural immunity because that discussion does not make Money for Big Pharma and Socialized Medicine. In our modern world, Greed and Money have replaced Choice and Honesty.]