Because they wanna fleece you for every penny you have, but the point still stands that for the most part it's to protect baggage handlers. If you wanna take a dead body on board you have to take that up with the airline.
If you are there handling heavy items all day, you're gonna get injured. That doesn't just fuck up the workers (meaning they can't go to work) but it costs the company money too (because they need to replace those handlers).
Think of it like this, you know how the X-Ray tech has to go into a little shielded room while you are getting X-Rayed, but you are still exposed to the full force of the X-Ray? That's not because they care about their safety more than yours, it's because one x-ray is fine (what your getting), but the x-ray tech does tens, maybe hundreds a day, and that builds up to radiation poisoning.
Now replace "X-Ray Radiation" with Repetitive Strain and back injuries" and "X-Ray" with the Suitcase full of Drugs your smuggling into the Netherlands, and you get the idea.
Now every single bag is overweight and they have to lift all of them all day. I suspect a few is ok but they really want to discourage too many of them so they are not lifting too many heavy weights all day.
I believe that after 50lbs it becomes a team lift and requires 2 people to lift the bag. Which 1 person can do but when hunched over in the belly of the plane it becomes harder and more likely to injure one’s back.
You know how X-Ray Techs go into a little room to protect themselves while you get your x-ray? That's because while you are only getting one (1) x-ray, maybe two (2), they have to do tens or hundreds a day, and the raditation levels from that can cause them harm, while you getting one or two x-rays is fine.
Now replace x-ray with your overweight bag filled with cheap Scotch you got from the Duty Free at EDI and "Radiation" with knee and back injuries from doing that over and over again for eight hours a day, and you can see why both the Unions and the Companies would object to unlimited baggage weight for passengers as workers will be out of work from injuries and companies will have to pay them compensation and find someone to replace them.
If individual passenger weight was enough to prevent take off, then very few people would be able to fly.