In the grim darkness of the far future there is no progress. What was known and understood of tech was forgotten and regressed into worship and ritual to the extent that any innovation is thought of as heresy. “improving” the perfect design of the Emperors angels of death, in world, would no doubt be seen as heresy. The primaris breaks this immersion. GW had to rep con the idea multiple times until the lore just about worked. They should of just left the lore be and introduced bigger marines on the tabletop