At the end of the day, regardless of anything else, it's titled a Final Fantasy game, so there are going to be some standard Final Fantasy tropes, especially when it comes to game world/game paradigms/lore.
One of those is people stand still while casting. In classic FFs that was thanks to the way battles were fought. Newer FFs do it out of a combination of lore and tradition. Expecting FFXIV to be any different just because it would make combat more complex (an arguable statement to begin with) is silly.