Because “My advice is get vaccinated and my insight is that I understand what they are saying and they probably do have some immunity but what it appears from studies because we are actually looking now at the antibody levels, at the T cell responses, in these folks that were previously infected and while yes you have some immunity, the amount of your immunity is less than if you get vaccinated," Pate said. "The duration is less than if you get vaccinated and the ability of delta and who knows what is coming next, but the ability for delta to get around your natural immunity Is even higher than the ability to get around vaccination. So, I don’t dispute you have some protection, but you don’t have great protection.” -- quoted from this article:
https://www.ktvb.com/article/news/local/208/idaho-medical-experts-explain-difference-between-natural-and-vaccine-immunity/277-97a4b3f5-a97a-4b12-9877-c8374d782453