Akshully, a significant portion of them prefers to fight without a helmet where possible, for various reasons such as chapter culture or certain gene seed variants. The primarchs are also often depicted without helmets, but considering the lore is essentially imperial propaganda it might make sense to depict them that way for PR reasons.
Wearing insanely heavy armor only to leave the helmet off seems like the kind of thing so obviously stupid it should be kept out of propaganda material at all costs.
Akshully, a significant portion of them prefers to fight without a helmet where possible, for various reasons such as chapter culture or certain gene seed variants. The primarchs are also often depicted without helmets, but considering the lore is essentially imperial propaganda it might make sense to depict them that way for PR reasons.
Wearing insanely heavy armor only to leave the helmet off seems like the kind of thing so obviously stupid it should be kept out of propaganda material at all costs.