You are forgetting the whole part where this is a videogame, so the point of playing is to have fun.
The reason there are so few tank players in contrast to other roles (even healers) is because tanking is not fun when you feel like your character hits like a wet noodle and is at the complete mercy of other party members to even take down trash mobs, let alone adds that spawn during a boss encounter.
WHM can obliterate trash mobs with Holy while healing. Why can't a PLD do the same with some useful AoE damage? It's poor game design and shows the developers don't understand why people play melee characters. They gave us special lion and bear mounts instead of just upping damage, which would have addressed the issue.
Not to mention, for a PLD to even get to end-game tanking they need to go through levels 1-60 of soloing quests, many of which require the PLD to fight groups of enemies during story battles. They are extremely frustrating experiences due to low DPS output. Compare it to, say, a Dragoon or a Summoner or White Mage who can clear these same challenges with absolute ease.
