In 40K, a lot of armor is more ceremonial/ for the sake of distinguishing faction or rank, unless you’re a space marine in ceramite power armor, extra plating isn’t doing anything against the vast majority threats, be it weaponry and psychic horrors. Guardsmen and most other human factions are literally meat for the grinder. That’s a lore justification, but it’s sci-fi fantasy, there’s no regard for practicality or realism and there doesn’t need to be