There are two reasons: fear and/or selfishness.
Fear... of the unknown, of the rumors, of becoming magnetic, of your dick falling off, of the government cloning your DNA into a pedophile pizza, or just of needles.
Selfishness... by thinking that the vaccine is about themselves. It's not. It's about their parents and neighbors and local business owners. Remember that "herd immunity" thing all these people were hot n frothy about when Trump said it? That's precisely what the vaccine does. Choosing not to stop the spread of the virus is putting yourself ahead of your family and the economy. This current round of mask mandates and entry requirements is entirely the fault of people being too selfish to get a jab.
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BTW, if you're one of the people that's afraid and are being hard and tough on here to overcompensate for that, just drive to a CVS or Walgreens in another town and get your jab. Don't tell anyone, pretend you didn't, nobody has to know. You can still be tough and macho. But you'll sleep better at night, because whether you're blue or red or purple nobody wants to get sick no matter how hardcore they are or who they vote for. Nobody has to know.
This is so right that I thought it was important to reiterate.