Again, a naive account of human psychology.
It's not just dumb people who exhibit biases that make it hard to change our mind. It's a human condition.
I've been in debates with very intelligent people who have gone down a certain rabbit hole that I find unreasonable, and it can feel mind boggling that they can't see the "errors" you are pointing out. But they are highly intelligent.
In fact, that tends to be just how it feels to be on the other side of a debate. Intelligent people can disagree on fundamental propositions, and it feels like your position can be so obviously correct "how could someone intelligent not see the problem?" But that's exactly what it feels like to the other side of the debate.
It's being human, whether you are intelligent or not.
And I suggest to look inward, and feel the appeal of the position you want to take: Is it mere coincidence that this is identifying a failing in "the other guy" that you aren't falling for? There's some nice, seductive self affirmation hiding in there, wouldn't you agree? It's well known that moralizing about others is an attractive stance, so I think it's wise to be suspicious when we are slipping to such characterizations. Not that we should never do so, or it's always unwarranted. But it's seductive and it shouldn't come too easily. (And yes I do it too).
Do you really think intelligent people can't come to believe unscientific things? Have you looked around at the world?