Second, the way one gets better at a skill in real life is by long, arduous training. Pretty much the opposite of "exciting fun." So if someone did create a game system that handled skill improvement realistically, no one would play it.
You'd be surprised. Some MMOs use a system like that for skills, and some folks seem to love it. I once heard a friend say something to the effect of "I only have to smelt 1600 more copper bars before my Smithing skill is high enough that I can start smelting iron bars".

Of course, that level of bookkeeping would be impractical without a computer, and smithing isn't really what D&D is trying to model in the first place.