when you think about it combining thousands of supersoldiers literally incapable of fear with a strictly atheist, rational regime was a colossally stupid decision, and not for the reasons you're thinking of

y'know those cheesy old horror movies where the protagonists are spectacularly stupid? like, they find a spoopy book bound in suspicious leather and go, "haha lets read from this WHAT COULD POSSIBLY GO WRONG" and twenty minutes later they're being chased by demon minotaurs or something. and the whole time you're screaming "NO YOU FUCKING MORONS WHY WOULD YOU DO THAT WAIT WHY ARE YOU GETTING UNDRESSED NOW IS NOT THE TIME TO HAVE SEX" and then the demon minotaurs kill them while they're having loud passionate sex because god forbid horny teenagers should ever have fun. ok so hear me out, astartes and primarchs in the pre-heresy are the same as the dumbass teenagers, except for the sex-having part (i think they T-pose at each other instead)

think about it. THEY KNOW NO FEAR. they're missing one of the key instincts we possess for recognizing dangerous situations. most people who find a spoopy book bound in suspicious leather are gonna be, y'know, spooped spooked. they won't want anything to do with it. the more anxious/scared will straight up leave. but astartes literally can't get spooked! they might burn the book for being xenos filth or something but beyond that there's nothing to balance out natural curiosity and/or lust for war trophies.

and this combines really badly with the imperium's rationalism! see, even if you don't fear the spoopy book, superstition/religion/fairy tales/etc. still provide a powerful incentive to stay away from it. you're gonna look at it and go, wow, that book looks satanic, messing with it might summon demon minotaurs, i should stay away. but the imperium actively discourages that line of thought! there's no way the spoopy book can do any harm, it's just a spoopy looking book! in fact, you SHOULD read the spoopy book just to demonstrate how not-superstitious you are!

yeah so this is why fulgrim walks into a creepy temple full of ritual snake orgy and thinks, hmm that snazzy sword the snake guys are obsessively fucking around would make a nice trophy. or why it never occurs to magnus that making deals with the fair folk might be dangerous territory.