It's not just that. And it's not stupidity or mental illness or whatever.

It's a world view/model thing. We cannot experience reality outside of our conceptual model of the reality.

For a large portion of the population, like the lady above, the model of government institutions, scientific expertise, the media and the elite in general is a very child-like and naïve thing. Their conception is that the government exists as a gentle parent tasked to take care of you. One who is guided by rational guidelines created by the top experts of their field diligently working to uncover the truth. Which truth will be spread by the media for the betterment of all.

To face the truth that your conceptual model of society is wrong would mean a literal mental breakdown. For the lady above to say "hang on, don't vaccinate your children" will mean to admit "every single thing I think I know about how the world works is completely wrong"

This admission is not an easy thing for anyone. And some people are simply incapable of it without guidance and support.