The reason why is that classicly trained craftsman/artist/scribes were taught to make letters between a series of parrell lines. Keep in mind that having a printing press (though in use for many years) was 'hi-tech' and not all could afford to use one.
I took a class at KSU called Lettering, Layout and Design. Oh, how I despised that class! I had the most terrible time doing the various fonts by hand. There were a few gals in the class that had such good hand skills, this stuff was easy for them.
Layout and design work was no problem, lettering, however....thank god I don't do it for a living!