Making a simple GTI device is rather easy.
Making a safe, electrically clean, efficient, reliable, cheap, and legal one is much much harder.
Accumulating all of the information that a person would need to understand, and implement in order to build a device that meets all of these requirements is even harder still.
Just because a person has an education in electronics and electrical theory its not actually the same as having real life experience with working with fabricating and trouble shooting it.
All the theory and simulation software you can buy still doesn't equal real life hands on experience.
As someone who has built many of them, trust me, simple, illegal and rather unsafe but reasonably efficient GTI systems are cheap and easy to make. Adding everything else thats needed is where the real work comes in.
Getting all of that other information directly from someone who knows the stuff beyond the simple, unsafe, and illegal part is nearly impossible!
For some twisted reason people with that knowledge just seem to think that everyone else should have to work for it too!