And few RTS games require as much multitasking as MMO games. Monitoring literally over a dozen cooldowns, tracking your own health, the health of potentially another half dozen players alongside your own, plus any pets anyone might have, keeping track of every enemy in your general area, which gets exponentially more difficult when PvP is viable, managing aggro for NPCs, and who they're currently attacking, requesting help from other players and watching/listening for requests from them...
And those are just the basics that work for almost any MMORPG (most of which are also applicable to non-RPG MMO games), regardless of class, and ignoring the class-specific extras.