Because the devs don't want players to put their rotations in a macro. Marcos are there for QoL stuff like Swiftcast + Raise + chat message or mouseover healing or quickly switching between hotbars etc. And from personal experience I can tell you that you don't want to have your rotation in a macro because it will get messed up if you need to move and if you play caster it will prevent you from doing anything else until the macro is over because cancelling spell #1 won't cancel the entire macro so it will begin casting spell #2. Or that is how it worked when I tried it quite some time ago. My idea as a healer was to put my dps rotation in a macro so it would only take one button and I wouldn't have to switch hotbars... horrible idea