I think it's the fact that vaccines are heavily pushed on everyone, which is very different than a medication that you choose to take, or your doctor specifically prescribes for you, to address a medical condition. Taking chemicals to cure some disease is different than taking chemicals when you're healthy simply to decrease a risk that you may or may not consider relevant to you. Also, often vaccines are administered not for your own sake but to protect the community, and so again it's a different calculation where you're injecting chemicals (or injecting your kids with chemicals) not because you think you need them but to potentially protect others should you become ill. Also there often isn't as much explanation around vaccines (besides COVID) as to their makeup, etc., compared to some medication that your doctor prescribes to you (i.e., if my Dr prescribes a blood thinner I'm going to get a pretty thorough explanation of it and can ask questions, whereas with vaccines it feels mandatory without much explanation other than "the surgeon general said to".

So it's not at all surprising to me that there is strong resistance to vaccines from some people who have no problem with medications in general.