So for rims this is also sometimes set, e.g. 90kg
Would you be worried if in that case you are at 70kg?
That's rubbish.
What does the limit tell? It tells that the 100kg rider shall not buy the lightest stuff and expect it to work perfectly fine. Weirdly, that's what they do so often. "I must buy the lightest stuff,because I am so heavy already". I don't get that logic. These riders must buy the stuff that can survive their power output , which probably is much higher than for a flyweight rider.
Why should a 50kg rider need to have material that was designed for 120kg?
I'm more interested in how those manufactures determine the weight limit.
Base on failure report? Or real proper stress analysis?
If just based on failure reports, it can be QA problems (or the next possibility) and you won't know if you're lucky enough to hit a bad batch.
Weight limit will reduce the chance, but you're still unsafe even under the limit.
And in that case there should be a recall, but everyone knows there will be no such thing for some random Chinese components that are only mostly targeted for domestic market.
If it's proper stress analysis, then it should be there from the beginning. Adding a limit later means there is very thin design margin.
In that case a weight limit will definitely help, and a lighter rider has nothing to worry.
But are you sure which case it is?