Because deep down inside, they are uncertain and unconvinced. They probably read the article you posted and wonder who the fuck is Elizabeth Weise and what exactly is this 'new evidence' she references that I should randomly believe in. They've become desensitized to the 24/7 impending disaster news feeds while watching Florida do nothing in regards to lockdowns, closures and mandates and have yet to see the state turn into a zombie apocalypse and pretty much experience the same level distress as other states and provinces.
Contrary to the annual flu shot example, they've probably never taken one (I never have). They are wondering why they should believe Fauci today, when he outright lied or was wrong when he said in an interview sometime ago that by taking the vaccine, it would be a dead end for the virus - and wasn't, not even for the other strains. And to Cloudsurfs question, they may not know anyone with adverse reactions to the vaccine, but they also know of little if any people directly around them that have died or become seriously ill. And they realize that covid deaths get recorded immediately, but real adverse reactions or deaths associated with the vaccine will take some time to catch up.
They wonder about these stats people throw around, like 98% of doctors support the vaccine, and wonder, is this 98% of doctors who are willing to speak? Over the past 18 months, I've spoken to 4 doctors in a professional and personal capacity. Two out of the four told me they wouldn't take the vaccine. My guess is the two against it (and who knows if they've since taken it), would never say this publicly. We know who gets 'cancelled' when they speak against, not the majority, but the loudest for a cause.
And lastly, most Americans (and Canadians) generally do the right thing, until they are told to.
Oh, and they watch videos like this that claim it is supported by a large swath of doctors and they don't know if any of it is really true, but think it sounds compelling.
www.canadiancovidcarealliance.org