Actually, Arcane’s rotation not simple at all, basically because you don’t have only one. You alternate between burn and conserve phase, you don’t rely on procs save for the occasional arcane missiles during the conserve phase.
If you arcane missiles spam at 120, you’re not playing it right (although it works while leveling, but there you could smash your head on the keyboard and it would work).
Arcane requires you to plan when to burn and when to rest, almost 0 reliance on luck. Sure the overall rotation(s) are simple, but you have more than 1. Warlocks have 1 rotation, slightly more complex (1 or 2 buttons more?).
If you add to mage’s rotation the aoe rotation and movement management with shimmers, then you can see how complex it gets.