>Are they just never taught to lift things correctly?

Part of it is that you find yourself having to lift in awkward positions often while you're having to carry heavy materials through obstacle courses, up on ladders and down stairs in ways that you'll inevitably have to compromise optimal form all while the wage clock is ticking so you have to hurry. In weightlifting you're afforded proper time and place to practice good form. That's coming from a Carpenter for twenty years and a weightlifter for four. Carpentry and a vehicle wreck pretty much destroyed my back but weightlifting and what I've learned from it has helped restore it somewhat.