Because some places, like Jamaica, the vaccine is not readily available. And in places where the vaccines are readily available, not everyone is smart like you and has been vaccinated -- and even in those places, the unvaccinated tend to hang out unmasked in clusters and communities of the unmasked and then unvaccinated and that's NOT the same as a homogenous mix (that's often assumed when "herd immunity" is mentioned).
Because, like second hand smoke, what people do around me affects MY health. When I see someone unmasked in a place where distancing is not possible, I have to assume (for my safety and that of my family and my co-workers, etc.) that that the umasked person doesn't care about others and is probably unvaccinated as well, and probably hangs out with other unvaccinated and unmasked people. Many of those assumptions are probably wrong more often than not -- but I have no choice. I wear a mask more as a courtesy for others than I do to protect myself. The "others" include staff -- some of whom may have family too young to be vaccinated.
I try not to name call -- notice I use "unvaccinated" instead of the terms that I normally use in conversation.
People ARE entitled to their own opinions. People ARE NOT entitled to their own science.
Bluntly:
If you don't want to wear a mask, you should stay away from others.
If you don't want to wear a mask, don't go to places that require them. Much like wearing a shirt and shoes in an establishment with a sign that says "No Shirt, No Shoes, No Service".