>>96770061
Because the Emperor was a prideful man and couldn't see the flaws in his own plans.

>The Imperium is an Under One Rule + Oppressive Autocracy build that had their leader fail and now is playing a massive and very wide empire but has almost no research and empire size penalties are killing them
>Necrons are basically just a bunch of Fallen Empires and a couple of them just woke up and soon there's going to be a War in Heaven
>Dark Eldar are some Barbaric Despoiler build that decided to play with some ultra-tall build like the ring world origin or something and are just trolling the other players
>The Eldar are basically just a collection of unusually strong Enclaves at this point trying to help the other factions survive
>The Tau are a Knights of the Toxic God ultra-science rush build trying to survive to get to Fallen Empire tier technology and hoping they can survive long enough to get to that point
>Chaos is the various different Unbidden invasion factions fighting the galaxy and each other
>Tyranids are the Prethoryn Scourge who are also invading at the same time somehow

>>96747417
That really doesn't work though when Space Marines are the equivalent of much, much more common enemies.