The numbers that are displayed in the menus are not your actual stats. If they were, everyone could hex edit their actual HP to 9999. Thus, the coding has absolutely nothing to do with it. The menus merely display what the server tells them to. The only additional coding they would have to do is to simply tell the menu to display the new number. They are either lazy, or want to maintain the "mystery."
That is a great point Josiahfk, never thought about the support from npc's wearing off. I was mainly looking at furnishings skill + and to actually know how much they give.
Correct me if I'm wrong SE left it this way so you couldn't get a ton of + skill items and progress to the next craft rank.
In the end, it's a lot more coding SE needs to do and that coding time is better spent elsewhere. For example more coding would be needed to not count + skill items in maximum skill limit of main craft and subcrafts.