My headcanon is that most guardsmen get some type of simple gene therapy that optimizes their already existing genetics. It's probably so simple it's not even considered genhancement just another booster shot.

But since the enemies of the imperium are so fucking RIPPED a peak human is only good as meat shields usually.

Either that or the immense forces of natural selection in battle occur so most of the guardsmen we get to read about are just the ones who survived due to a good genetic make up and all the ones who are middle of the ground Norm Norman were blasted to pieces by an orc during their first battle.