why don't I just say "I don't know"?

Truth Sandwich (George Lakeoff, 2018?): trying to refute an idea can spread it, because your audience is probably not the audience that was first exposed. Another name is prebunking (but committing publicly to a position on something has dangers: if you turn out to be wrong, it will be harder to admit that).

Defensiveness: its embarrassing to be wrong in public, so once you have publicly committed to a position its hard to change your mind.

Confirmation Bias: Most people are much better at making up arguments for something they want to believe or against something they don’t want to believe than at finding the truth. This is one reason why academia is based around debates.

Confabulation (Skeptic’s Dictionary): see also Power of Fiction. NB. “children and many adults confabulate when encouraged to talk about things of which they have no knowledge.”