Because most game tutorials are about moving things around, maybe sounds, maybe collision detection.

And the actual mechanics of, notably, a 2D engine, are reasonably straightforward. But going from moving stuff around to “game” is a lot of work. A lot of polish, testing, feel, balance, etc.

Now, to be fair, a full tutorial of something like Breakout or Space Invaders doesn’t seem like too much to ask, since they’re so simple. But then it depends on what kind of games folks want to write.

Something like a simple RPG or platformer can be complete black holes.