The premise is pretty silly and indicates the author doesn't really understand the history of 40k's lore. Between real chaos gods corrupting any civilization too psychically involved with the warp and civilizations entirely unable to be reasoned with like the Necron or Tyranids, the imperium of man's fanatical xenophobic culture is pretty reasonable. The only civilization that could have perpetually positive relations with humanity, the Tau, are on a path towards artificial general intelligence which humanity fears could lead to another near-extinction of all life.

The emperor didn't even want to be worshipped. That only really became tolerated and encouraged after the horus heresy once he was locked away onto the golden throne, and then-fanatics had nobody to keep them in check as they pushed their own narrative.

One could argue that humanity could welcome xenos into the empire while remaining at war with the other civilizations, but I tend to think they find it much safer to not take chances when other races have natural affinity to the warp and could be vectors of further chaos corruption leading to extinction.

It's because power armor and bolters are cool as shit, just like why people join the Brotherhood of Steel in Fallout.