Because they will reduce their already low compliance with best practices for avoidance of contact with the virus. They will feel "safer" since everyone else CAN'T get the virus, that there's less chance for them to contract it. So they will relax their precautions. (As will the 50% of people who are asymptomatic spreaders.)

And even if they recover from their bout with COVID, there's the permanent lung and brain damage, as well as the tendency for major organs to suddenly have blood clots. And, the younger they are, the longer they have to live for one of these knock-on effects of COVID to kill them before they "should have" died.

It'll be a cleansing of biblical proportions. It only takes one person in a crowd to spread it to hundreds of people and kill dozens. And if you are already an anti-science anti-vaxxer, you aren't even adhering to the most rudimentary protection measures... and are probably having other anti-vaxxers over for coffee and conspiracy-watching sessions on YouTube with other people with poor hygiene... and that's just a matter of time before everyone in that "club" get it, and has lifelong health issues from it.