A number of things, I guess;
- You need a part that doesn’t otherwise exist. (Where do you get a stock steering wheel for a 72 Pantera? You make it)
- Superior quality and authenticity over existing after market or kit based alternatives. (Pre-wired distributors... nuff said)
- The pride and satisfaction of doing it yourself. (Opening the hood to display all that cool PE, resin, and 3D print is really a show of how well you fitted someone else’s work)
- Biggest reason; it’s fun!
Those are my personal reasons for scratch building. Not knocking anyone else’s methods. There are much faster and easier ways sometimes, but in this way I can take full pride in the work I create.