What stops airlines putting a total weight limit of person plus luggage on?

Are you looking for some satisfying and perfectly logically consistent explanation to this, across all airlines? Because if so, that’s not how any of this works. It’s just a series of compromises and practicalities made by different private companies which are ultimately just groups of people who will have different priorities. Some airlines care more than others. There’s no requirement that a company policy has to be logical, it just generally has to not be illegal and be clearly explained to people before paying. Most airlines settle around similar limits because they work fine for most people and so there’s less surprised tantrums, but still leaves room for people who want more and will pay. They could mandate that you have to hold a banana on your head for the duration of the flight, or pay an extra $20 no-banana handling fee if they wanted to. If that’s what the policy says then that is the policy and there’s no debate to be had. There doesn’t have to be some mountain of philosophical underpinning to it.