I don't actually know why this is so hard for people to grasp.
"Hillary Clinton does not eat child brains in the nonexistent basement of a particular pizza place" is not a statement that "Hillary Clinton is a great person and we should do nothing but sing her praises."
But people will climb out of the woodwork to explain how she's "bad" when you make the first point.
As if defending her against a spurious accusation is about her and not about affirming reality.
Saying "a test that looks at bacteria going through a mask onto a petri dish is not a fair way to evaluate efficacy against an airborne virus" is not a statement that masks don't work.
It's trying to ensure that when the evidence comes that masks do work that evidence can be believed and relied upon, and that you haven't destroyed trust with people so that you can communicate it.