"...why otherwise rational people defend the absurd notions..."

Because humans have to ability to suspend rationality at will if it suits some deeply held idea.... whether it be a childhood teaching, adult prejudice, cultural 'norm'...etc.
Look around you at the dozens of ways rationality is suspended in daily, simple ideas & acts. Simple superstitions, personal habits about food (eating things that you KNOW are bad for you), taking chances in crossing streets...(rationalizing that "I will be careful, and I am nimble")... it goes on.
There are **NONE** of us who are entirely rational on all things at all times, and no army of psychiatrists can either explain or cure the myriad ways our 'higher' thinking abilities manage to convolute and rearrange themselves in order to defend what others may call "absurd notions".

An American columnist, Sydney J. Harris, used to do an occasional piece where he'd define attitudes in terms of viewpoint... as in:

"I have the courage of my convictions. YOU are a bit stubborn. HE is a hardheaded fanatic."

How to approach the someone who is using what YOU feel is "absurd notions"? First, you MUST be able to comprehend how they got them and whether they are serious. My ex-wife, (a brilliant genius on some things) used to use the heel of her hand in the bathtub drain to "make the water go down quicker". The 'sound' made her 'feel' like it worked, and she had done it as a child. A gentle, short explanation about how any pressure she added to the process was negated by the time she had the exit covered was all it took to convince her of the waste of time. I very much doubt that calling it an "absurd notion" and telling her she was being ridiculous would have helped.

I FULLY understand how religious ideas...and many cultural ideas ...were instilled in human history...and unlike bathtub drains, cannot directly be addressed and/or disproven. I do know that insults seldom produce the desired response. (Jean Redpath, the Scottish singer, once said: "Did you ever notice how, when you have a heavy accent, people YELL AT YOU S-L-O-W-L-Y?")... a similar, useless attempt to deal with a situation.
Yes... I know how frustrating it can be when you are SURE you have explained it clearly and someone seems to just be unWILLING to comprehend. I have a dozen topics where that happens. I can either: A)go away and give up,B)YELL AT THEM SLOWLY..(with insults), or C)keep patiently trying in hopes that my basic logic is gradually seeping in, whether it shows or not. D,E,F?? maybe...but most other attempts are only variations on A,B &C. In my case, because of my training in logic & philosophy, it suits me to practice my explanations.

"..and who knows, perhaps the horse will learn to sing."