Maybe because even though all Pokemon were created by Arceus, not all moves were created by him.
Let's take a real life example: God made humans, but God didn't make cars, guns. buildings etc. these were all made by humans. Similarly in Pokemon there's a chance that many moves (like Octazooka, Volt Tackle, Transform, Sketch) were all originally developed by the particular Pokemon, and not gifted to them by Arceus.
Then again, all theory, there's a million more possibilities out there.
Hope I helped!