Why is installation rather than fabrication & embly of windows and doors the subject of training courses?

Because no matter who makes them, installation is pretty much the same and requires no specialty tools. This isn't the case with the fabrication and embly aspect. Depending on brand and manufacturer, the crafting method is going to be different, and may require specialty scribes, lathes, or big machinery, not to mention the varying materials used (vinyl, aluminum, wood, steel, etc.) and styles.