You are treating a written product the same as a commodity. The success of a creative work isn’t assured and fixed like a commodity is, basically working like fiat. So a piece of writing can either make very little money or a bunch. The initial cost of creating a show would be so much higher if writers were paid based on what the script is “worth”, since there is no real metric for show success and writers would probably just charge up the wazoo to be able to survive long enough till their next gig. This would probably also decrease creative quality, and you’d probably have shittier shows because of this.