You keep acting like the emperor is infallible, when the entire point of the lore is he’s a crazy genocidal mostly dead guy who wandered around conquering and having people worship him as a god.

You don’t need an explanation beyond “Emp was a space fascist, he thought girls were icky and only made boys into marines, so now they keep doing that because everyone still follows his wacky shit without thinking because they’re in a death cult. In this universe the supposedly technologically informed techpriests mostly do maintenance because they think a soil inhabits inanimate objects and that it will be angered if they don’t give it sacred oils. You don’t need a scientifically coherent explanation, and you don’t need to prove it’s not discriminatory, the Imperium is a comically discriminatory genocidal empire it would actually be weird if they were tolerant and progressive.”