>>96492608
> the creators never actually thought about any of it
yes they did, there's multiple references to it since RT.
>>96492610
> making aliens develop in complete isolation identically to humans for no reason
See above. And the trope has a built in explanation seen in other IPs: they all have a common ancestor. That's the case in Star trek, Stargate and a bunch of others.

Hate this trope all what you want, it doesn't make it less real in 40k