If I do a cost/benefit analysis on the booster, the cost is pretty trivial... just a little needle poke while I'm at the grocery store and maybe a few little side effects like a slight bit of dizziness, which is what I experienced from the first two shots. The benefit can vary from none, if I'm never exposed to the virus, to life saving, if I am.
So for me, it's an easy decision. When I get the shot, I'm not going to be ostracized for going against a group's ideology since I don't belong to any groups and I'm already ostracized for not being friendly and bubbly around the neighborhood.
For other people, they don't want to be excluded from their anti-vax, anti-mask groups, which are also their political groups, so the cost is much higher, albeit it's purely psychological since the vaccine has proven to be safe for all but less than 1/2 a percent of the recipients.