WH40K uses the idea that Races CANNOT manufacture their own tech, so they rip from gigantic hulks of old battles and stations the tech necessary to keep the remnants of their empire alive. Its a post-apocalyptic setting in space. The idea that "the Prime Directive is violated" seems a theme in Traveller, but its a theme missing from Warhammer 40K. Well, there is one singular moment of it occurring, but the person responsible dies and thus is the reason for the post-apocalyptic feeling to the setting.
If you were playing a race in an "post-tragedy" setting, the idea of solely relying on Archaeotech might be an idea. But Traveller's emphasis on trade and travel keep unity where empires do not, makes it very hard to ever hit the point WH40K STARTS in. Emphasis on starts, you literally start in a setting where everyone is only competent enough to keep the old machines functioning, they cannot nor know not any way to make new ones. If you want Mad Max, that's probably where you would wind up in RPing eventually.