When you're doing an electrical job, you might have to put your body in strange contortions that don't follow any harm reducing "correct" forms.
There's your speculation and incredulity, and then there's all the people I've talked to who do it. They disagree with you.
I found out when waterskiing that you don't get strong waterskiing, you get injured. You have to spend a lot of time in the gym so you don't get injured waterskiing.
Back when I was younger in Seattle, lumberjacking was still a thing. They told me that the limiting thing was injury. They'd work into their sixties. There aren't many jobs harder than lumberjacking. I wouldn't mess with any of them, either, they are freakin' strong and I'm sure they could cleave me from head to toe with a casual swing of the axe.
I'm not in the least surprised that people working in trades mess up their backs, nor do I doubt that they do. But injuring one's back is not the same as wearing it out.
Edit: Downvote me all you like. I've done yoga, I've done plumbing, I've done electrical. Yoga takes the cake by a mile for most painful contorting.
And yet I know an 86 yo woman who has been doing yoga 3 time a week forever and is fit and sharp.