Storytime: When I was a teen, I worked for a moving company. When a box was over 50lbs, (~22.6Kg for you Europeans!), there was an exorbitant fee. This fee, supposedly, went directly to an insurance policy for the employees. The idea was that if we are lifting heavy boxes at risk of injury, then at least we would be protected if our backs gave out.
This policy is likely similar or identical to the policy of the airlines. Airline employees have to lift heavy items all day, and a generally agreed upon limit for heavy items is roughly 50lbs/23kg, as determined by medical professionals. Airlines could be at risk of union or employee retribution if they forced employees to engage in activities with serious health risks.