Because that’s how a society functions, and that’s how vaccines work.
When a vaccine is effective against a pandemic, it prevents it from spreading. If there are not enough places for the virus to live, or it can’t get to a new place to continue living… it dies out.
There is a tipping point where enough people have been vaccinated that the virus can’t proliferate… which means people at risk from rare side effects of vaccination don’t have to be vaccinated.
People have undiagnosed health issues and end up in hospital from roller coasters or from tying their shoes.

Like how deforestation drives species to extinction. There are still forests around, but the species that go extinct can’t get to another forest and end up dying.
Except vaccination has the positive effect of saving lives, whereas deforestation just ruins ecologies.