Honestly?
Injections are scary and painful, it's horrible to have to hold your baby/toddler down while they get jabbed uncomprehendingly, and when you feel emotional about something (fear being one of the more primal/less rational emotions) you are very susceptible to latching on to anything which gives you an "out" or makes you think perhaps the scary thing that you don't really want to do might not be necessary.
There is an industry that latches onto this kind of fear and uses it as a starting point to hook you into their agenda - the whole "alternative health industry".
If you look at a lot of alternative health stuff, it's all cleverly worded to prey on fears and doubts about "mainstream medicine". This is because the more people who doubt mainstream medicine, the more people will search for an alternative, and that's how they get their customer base. It's very insidious.
Now, I have no doubt that some people lap this up because it makes sense to them, because of some previous poor experience with the medical profession or an underlying fear or whatever it is, and they believe it and pass on the information in good faith.
It doesn't stop it from being a very tempting narrative.
If you're feeling really anxious about having a vaccine or taking your child to have a vaccine, and somebody tells you well, you know you don't have to do it, right? And it's not really that effective anyway. Your immediate response wouldn't normally be "shut up you nutter", it will be "What do you mean?? Why not?" and if there is a plausible-sounding answer that gets you off the hook, someone feeling anxious or afraid is very likely to be intrigued by that and go looking for further validation. And thanks to the internet, it's incredibly easy to find. It doesn't matter at this point that all of the antivax sites look shoddy and amateur - this is in contrast to the official health bodies whose websites are very slick and well designed and polished. That actually serves the antivaxxers well, because it comes across as "We're the real people, you can trust us, that's a shiny corporate front."
If you don't feel anxious about medical procedures or needles, perhaps this sounds completely off the wall to you, so try imagining it with any scenario or situation that you absolutely dread. Maybe it's public speaking, or job interviews, or flying. If someone told you tomorrow that you never had to do that thing ever again but it wouldn't involve any sacrifices or loss in your life, you'd jump onto that suggestion, wouldn't you? You wouldn't just dismiss it as rubbish. You'd want to know more.