The idea of a tank for the trinity is that enemies fight the tank because they must, not because they want to. A lot of times MMOs portray tanking with taunts, like a generic yell. Another way to think of tanks is like a grappler. If you try to run by to kill the healer or the mage, the tank grabs you as you run by and holds you so that his allies can kill you. You fight the tank because you can't ignore them safely, not because you want to fight them.

Most MMOs don't have precise directional attacks. If they did (and better positional code than FFXIV), then tanks could get targets of opportunity on anyone who ran by or wasn't facing the tank. It would achieve a similar result, that enemies would need to face the tank when the tank gets in range and be cautious about disengaging.

The problem you point out is that most tank skills look like they can be ignored. That's an unfortunate portrayal, not a problem with the tank role.