From your continued asking of the "why" question, it makes me wonder if you've ever tried to build a DIY anything yourself. Developing something like this involves a lot of the designer's literal blood, sweat, and tears. There are all sorts of levels of DIY from using 2x4s to machine precision accuracy. I will say, that precision is what astro is all about though. A seemingly innocent +/-1 degree accuracy will be okay-ish for something like wide-angle DSLR type shots, but once you start using longer lenses, you will be wasting your time.