Why do most people think that using AI is just a simple thing
Because people want simple answers to comlex things. AI bad is a simple answer. And devalueing the work spent on an ai image is quickly done by claiming it is simple. I tried once for fun. I failed, or rather I saw my limitations. If I were to make a game I probably would use a 3d or 2d engine or use renders - and still no artists would get commissions...
Sure, if you know what you are doing, creating images is a hell a lot faster than digitally creating them with templates in your sophisticated image creation software and is hell a lot faster than painting it on paper or canvas.
And even if you know nothing of the other ways, using an ai tool will give some output. And good luck with consistency, if you have a character.
Those people probably also think that taking a photo is a simple thing. After all, there is no such thing as professional photographers, now that everyone has a high resolution camera in their pockets all the time. ;-)
It is just a different skill set. I guess a skilled photographer and editor would qualify for many things a good ai operator would need to know: spot and select the good ones.
about using AI: Does it devalue the game development process or the inverse ?
It devalues some games. But it heavily depends. Some games get value from AI. Some even get existence, because they would not have been possible without it. But that the most popular games rarely feature games with ai is not a coincicence or because of ai hate. I think it also correlates with general budget and professionalism. So, hobby games tend to have more ai and professional games less. On the artwork side at least.
As for the development process itself... how do you compare the value in that? If it gets you quick results and keeps you going, that is value. If it bores you or you are frustrated because you can't get a consistent main character with ai, that's bad and no value. You might learn new skills, which sounds like value. But you might focus on skills that do not advance your game development, which sounds like less value.
With game development in particular, there is the thing with game engines. Does it give value to the game development process, if you use an engine, instead of using basic os library calls?