>>96326000
>>96326345
>>96326701
It's one of those things that seems to be accepted as true in-universe, but as you say, when you think about it, it doesn't make a lot of sense. There's even the odd time when someone In-Universe will question it as well.

Like in the Iron Hands books, one of the viewpoints we see it from is of a scout, and the kind of crap they get put through in the name of "driving out weakness" is just insane. Basically trying to turn them into school shooters on purpose, and stuff like using live rounds on the scouts during training, so our scout goes through most of the first book short his left arm and lung.

Another of the MCs, in the second book, is sent to Mars to get Iron Father training, and there starts getting to know recruits from other chapters. He takes note of how they seem to do just as well in a fight as the marines of his chapter, and starts to wonder if all the crap they put their trainees through, along with the chapter's extreme loathing of weakness (which at this point in time is reaching such a level that a full-on chapter civil war is about to break out) actually works, or if they just tell themselves that it does.

Gulliman certainly doesn't believe it, considering his first command to Dante upon meeting him is to uplift the people of Baal from their savage existence once they're done clearing up all the dead nids, and it makes sense. Like, him and his ultramarines were raised amidst peace and plenty, and they don't seem to have done too badly out of it.

For my part, I'd say play it whichever way feels right to you, as there's fluff enough to back up whatever approach you'd favour. Perhaps the culture and lifestyle of feral worlds means that whichever soldiers *do* reach enlistment age are already hardened survivalists who can chew nails and piss rust, or perhaps this is just a culture-wide Fremen Mirage that the Imperium has bought into and there's no real difference in physical ability among its unmodified troops.