A couple of things you're missing: 1) Motivation. How many of these have you or anybody you know built and regularly use? 2) All the details. They require many skills, tools, experimentation, time, and spending of money.

As an example, I'm in the middle of a stalled project now where I want to make nice looking fishmouth pipe joints. I decided to cut them with a hole saw. But I don't have an electric drill that's slow enough so I've struggled doing it by hand and with a windscreen wiper motor, all the time wondering if it would be worth buying a drill press, but not the cheap ones from the home handyman shop because their minimum RPM is too high. I saw a cheap used one with no motor online so maybe I could somehow get an old washing machine motor and build some adapters to fit it together and buy an electronic controller - and that would be a whole new project in itself! What a lot of fiddly, costly details to "just weld some pipes together".