IT’S BECAUSE OSHA REGULATIONS! A SINGLE PERSON IS NOT ALLOWED TO LIFT MORE THAN FIFTY POUNDS! BAGGAGE HANDLERS ARE PEOPLE! YOU PAY MORE FOR HEAVY BAGS BECAUSE THE HANDLERS ARE SUPPOSED TO USE THE BUDDY SYSTEM TO LIFT THOSE! IF THE TOTAL WEIGHT MATTERED SO MUCH, PASSENGERS WOULD GET WEIGHED, BUT WE DONT QUESTION THAT NOW DO WE?
And like FYI if the plane is small enough they WILL move the passengers so the weight is evenly distributed.