vaccines are not 100% guaranteed to keep a child from getting the disease, but they do carry a risk of causing adverse reactions. Getting a vaccine therefore means a child can both have adverse reactions AND get the disease in question.
Not getting a vaccine just means they can get the disease. They can't be hurt by a vaccine if they don't get it.
I choose the 100% guarantee. I can live with my child getting the disease if I didn't vaccinate because they could have gotten it anyway. I can't live with causing my child to get a vaccine injury because that IS 100% avoidable.
I don't want my child to get sick. I'm not hoping she gets vaccine preventable diseases. I don't want her to die. I'd rather support her immune system so that it can be at its strongest should she get a disease though than give her a vaccine and think she'll be just fine. I've known people who's kids got vaccine preventable diseases and their kids were fully vax'd on schedule. Their doctors also refused to believe it could be the disease at first because they were vaccinated.