Because how are people supposed to know you are making a subjective/qualitative judgement? It's too easy to mislead people, and too many people profit from taking advantage of that.
A true argument can stand on its own.
A true argument can stand on its own.
If I say something potentially provable that I am unsure of but claim is a fact ("Metallica is the best-selling band of all time"), this would be misleading, and I would be taking advantage of my audience.