I genuinely think there are people who don’t “get” hobbies or, if they do, don’t get that other people might like hobbies other than the ones they like. As you say, the point is to enjoy it and that’s all.
Coloring books give you some freedom; to pick your own colors and texture. If we were to draw an analogy, it would be using the LEGO pieces in this kit to build something else. But nobody is going to do that, so it is like a color-by-numbers book, which exercises little creativity. It's a mindless activity to pass the time.
^ bizarre thought process tbh. Both building it and displaying it is fun. That's all.