What your argument misses is that do in fact buy WIP goods. Car kits are a thing, as are individual parts of cars. On the used market often people can break down a complete car or motorcycle and sell the parts individually for a good bit more than they could the machine as a whole.

Likewise, tracking the value of work in progress absolutely matters in a plant, and the value change at each step has to measure actual changes in value inputted to the product. You can't just say "This product has all the parts assembled and complete and is worth 100$, but once I put a 1 cent sticker on it it now costs 500$." There will be audits aplenty from unhappy people if you try that. By the time people get their diploma or bachelor's degree they have already completed all the class work, so the sheepskin effect suggests that the course work is not the valuable part. Otherwise people would just take classes and get grades, then tell potential employers about that, much like they do for professional certifications.