Why did the Emperor make so many silly mistakes? Was this truly all we could expect from the pinnacle of human potential and ~38,000 years of experience?
You know when you do something for so long that it becomes rote and you feel your own actions are obvious.
The emperor is that on crack. He's someone trying to uplift humanity who at the same time doesn't properly understand it's flaws and expects everyone to be held to a standard that's basically impossible
Those weren't mistakes, they were genius plans. Your mortal mind is simply too small to grasp the true breadth of the consequences of his actions. You only see the tiny sliver comprehensible to yourself.
Any puzzle would look strange with most of the pieces missing.
E-Money organized the Horus Heresy as a distraction so he could build the Webway. Unfortunately, Magnus fucked it all up and Emps wasn't expecting Horus, Fulgrim, and Alpharius to fall.
That only applies if the environment these actions performed in remains static.
If her really was an amalgamation of psykers born in Anatolia in ~8,000 BC then he managed to not get bogged down in bad habits until the last 2 or 3 hundred. And Unification/Techno-Barbarian era Terra was not similar to Straife, DaoT, Great Diaspora or First Stellar Exodus Era Terra.
it was when the great crusade got going. Before emps preferred to work in the shadows behind the scenes not be directly involved in guiding mankind everyone was worshiping the gods he made up not himself. when the great crusade started all the things that emps did were credited to himself and not those dieties and everyone start thinking of him as a god and started developing a god complex
Arrogance? After millennia of human misery across the galaxy, Emps finally gets Terran humanity united, but has to do it by force. He concludes that only by force can he unite all of humanity. He created the Primarchs not as sons, but as generals, and treats them as such. They're already adults, they can handle a few setbacks and rivalries; Emps doesn't need to care as long as the Crusade continues. With success after success, he concludes that his campaign has the momentum to succeed, so he appoints Horus as Warmaster and lets him take the reigns. He ignores the goings-on in the galaxy to focus on his Webway portal.
Then everything goes to shit because the people he believed could handle it couldn't and it's too late to step in and put the Crusade back on track.
The real reason for it is the same reason that 90% of Dieties and ancient beings suck in stories.
It is because they are omniscient infinitely wise being that are written by writers who are distinctly NOT omniscient and infinitely wise.
This creates the effect of, for example, making the emperor into a giant fedora tipper because instead of a perfect argument that can only be gained from 30,000 years of wisdom, it was thought up in 30 seconds by a mortal man.
Combine that with the horus heresy being a prequal and it HAS to include a few specified events and an ultimate destination. And you arrive at the fact that in order for these seemingly unrelated events to happen that everyone has to be a GIGANTIC moron or a huge dick. Or both. They're not mutually exclusive.
That is why. Bit meta. But that's why.
Because he was nowhere near as amazing as Imperial propaganda made him out to be.
Also, the Emperor is a metaphor for mankind as a whole. He embodies the best and worst aspects of humanity. As 40K is ultimately pessimistic, the negative aspects of mankind - paranoia, hubris, greed, xenophobia, entitlement, etc - eclipse the positive.
Basically, the Emperor a shit because mankind a shit. According to 40K, anyway.