The vaccines provide very nearly 100% protection from severe disease, which essentially means that anyone who doesn't want to get COVID can protect themselves completely by simply getting vaccinated themselves. When you have the ability to provide yourself with a very high degree of protection, why do you even care what other people do? If you are vaccinated, why does it matter to you at all who else is? Why can't we just let those who want to be protected protect themselves, and let those who don't want protection deal with the consequences of their own choices? It's so simple that I really can't even comprehend looking at it any other way.
Natural selection though. Higher numbers of unvaccinated individuals increase the number of available hosts. The more hosts, the faster a particular viral strain may become dominant and increase the risk of escape/lack of immune protect from the vaccine. Thus limit the number of available hosts by vaccinating.