Is it? I've watched a lot of modern car assembly line videos and it doesn't seem to be hard or backbreaking at all. Boring and tedious? Sure, here's one for GM's best selling vehicle: https://www.youtube.com/watch?v=_Nr3I-Fddv8
At no point do you see anyone lift something heavy without the assistance of overhead cranes.
That's a promotional video showing you what the most automated parts look like when automation goes perfectly. Not everything is so automated, and everything doesn't always go perfectly.
Sure, backbreaking work exists within the industry, but you can also tell from how some people are dressed (just street clothes with no safety gear) and by how fast the line moves that there's a lot of jobs that are no more physically demanding than some office work or fast food work.
E.g. it's someone's job to sit in a new car and make sure all the buttons and interface elements work, or to visually inspect the paint job and flag blemishes for fixing further down the line etc.