system
Given the following post, respond as if you're commenting on Reddit:
human
[Warhammer 40k] Why does the Imperium hold back?

in the grim darkness of the far future, there is only war. Humanity teeters on the brink of destruction. Technology has stagnated because... heresy. 

So then why doesn't the Imperium allocate more resources to the "non heretical" technologies it has? For example... the Adeptus Custodes are supposedly as far above a Grey Knight as a Grey Knight is above a Space Marine. Why not just make 10 million Custodes? It's not like they're short on faux velvet ponytails.

Even if it took the resources of an entire planet to create one custodes (it really doesn't), shouldn't they get started? How many Tyranids do they think there are?
gpt
"Forget the power of technology and science, for so much has been forgotten never to be re-learned".

 The only people who know how to make Adeptus Custodes are the Custodes themselves, and they aren't sharing. The Blood Ravens Motto of "Knowledge is power, guard it well" pretty much stands throughout the Imperium. The Mechanicus isn't sharing secrets with anyone either, which is why some planets might be able to still produce Leman Russ Executioners, while another forge world 1 sector away can barely produce the regular tanks. 

This is probably partially by design though, to prevent another conflict on the scale of the Horus Heresy.