Making the tutorial increases their profile, leading to more profitable opportunities elsewhere (because making the tutorial is not as financially remunerative as actually being employed/consulting somewhere).
Furthermore if you make a tutorial as an advertisement for your skills you want to make it as quickly as possible and show off skills that an employer will appreciate. Probably the quickest way to do something, or to decrease the cost of doing it is to work on something you want to make anyway or to adapt something you already made for a client/project. OF course the easiest way is for people to just go ahead and take the official documentation examples and tweak them a bit and now we have a new tutorial showing you how it is done and we have also managed to improve our profile! Everybody wins, only you don't win as much as the tutorial writer.
Because you want to show off skills to the employer you might also have to consider what the skills you think make you bankable - especially if you have a plan as to where you're going. So even though your GraphQl tutorial might not need much on the frontend you may want to throw React/Redux in there because
1. you're good at React/Redux so gotta show that off 2. more code looks more impressive to a lot of people.
Finally, When you make a tutorial on GraphQl you will need to have some sort of backend, frontend etc. so you have to choose something, probably you decide to put those together based on what your skills are, based on what you have that you've worked on that can be adapted to tutorial purposes, and a number of other factors.
At any rate there is not much economic incentive for most people who make tutorials to make the best tutorial for educational purposes that they can make, unless it is their plan to be educators in the future.