Over the years I have seen multiple MMO developers asked this question of why does the boss go after the tank and not the healer and the answer has always been the same.

"Why do we have orcs and elves!??!? because we can. It would not be any fun if the boss was actually smart and just ignored the tanks the entire fight"

FF14 also has mechanics you do not even realize that very often target the healer. All those stack mechanics with big yellow arrows?? That stuff is Square trying to change up the same old formula.