As a tank, you are the de facto leader, because you literally lead the charge.
Just because you pull the targets does not mean you are a leader. Technically, anyone could pull. In FFXI? Healers and bards(a support job) did the pulling, or in roaming parties even DPS pulled. It's the playerbase that decided in FFXIV that tanks do the pulling. At any rate, it's fallacious to say that leading a charge makes you a leader. War generals leading their armies did not ever lead charges, they were in the back. Generals and Commanders do not rush headlong into the frontlines. Tanks control the PACE of the group, but to say they lead everything is a biased view.