system
Given the following post, respond as if you're commenting on Reddit:
human
[WH40] Why doesn't the imperiam disintegrate?

My knowledge of WH40 comes exclusively from this subreddit and the Emperors Text to Speech series, so my understanding may be bad, but...

The imperiam seems completely unmanageable on pretty much any level. 

* Communication times to areas outside the inner most territories are measured in years or centuries. This means that any orders are basically unenforceable and control falls to the local strongmen controlling at best a segmentium each. This should just about finish the Minastorium off as an effective govenment.

* Oversight of the various organisations that police the imperiam is minimal at best. Combined with communication times this will very quickly lead to radically different understandings of orthodoxy and what the imperiam is. This should mean that various branches of the inquisition are forever at war with each other.

* This leads to conflict within the empires main bodies and between them. Given that the empire is fanatically militarised, the invariable result is full scale civil wars between, for example various marine chapters, or between marines and the inquisition. So basically the 'govenment' should be a clusterfuck of continously fighting official bodies and breakaway petite kingdoms.

* On top of this the imperiam is absolutely beset by powerful foes of all kinds. Unauthorised negotiations and weapons development would run amok as people try to cope with largely theoretical government support. There should be events of the level of the AI revolt and the Horus heresy every year all around the empire. Without a strongly co-ordinated response these will create powerful splinter factons with no regard for the failed emperor worship of their past. And that is basically impossible.

* Worshipping the Emperor would be severely diminished. Just look at the history of any long lived religion and you can find plenty of examples of isolated groups denying the othodox version. And this is in a world of travel times measured largely in months.

Basically the will of the imperiam is completely unenforcable in the near term and culturally fragmented. They might claim to be imperials, but really WH40 should disintegrate into a medievil Europe situation except with a whole lot more chaos induced darkness.

So given all of this, what exactly holds the system together?
gpt
The Imperium *is not* a culturally, socially or politically strongly coherent state. There are extreme differences between the planets in it.

Some are paradises with a nice democratic government, some are dystopic 1984 style hellholes where free thought is outlawed and punishable by death, some are overcrowded anarchic cyberpunk hiveworlds ruled by Mad Max like gangs, some are plain weird and overgrown with alien jungles and creature straight out of nightmares, or post-apocalyptic wastelands where people are grown in ancient cloning facilities deep below the ground to become soulless soldiers.

The Imperium is exactly the clusterfuck you are describing.

The only thing holding it together is the fight against its enemies. Governmental duties are left almost entirely to the individual worlds and they are allowed to do as they please. The only thing everyone has to do is pay the tithe and send regiments of guardsmen. And pray to the Emperor of course, but how exactly you do that is even up to you. 

It is more of a federation than an empire in most parts, being held together entirely because everyone in charge knows there is no alternative. Alone you get devoured by extragalactic space ants, smashed by insane mushroom people or massacred by lovecraftian horrors or their meatpuppets.