It's because jobs are thematic. PLD and WAR are not the same job. Sure, they could have had PLD with 2 specs. One spec that was defensive heavy and focused on blocking and some magic or a spec that was offensive heavy that built up a gauge for offensive attacks. And then come HW, they could have a PLD that ate up MP to buff their attacks and abilities. Then we'd have 1 tank, PLD, but 3 different specs,a ll the customization you'd want right? But you'd also only have 1 tank. What if that person doesn't like swords and shields? What if they don't like PLD's aesthetic? I guess they are SoL cause there's only 1 tank in the game, just pick a build you feel like using right?
OR, each one of those specs could be a job itself and boom, 3 choices for you to use, more that aren't the sword and shield you don't really like. In this game, jobs are essentially your specs while roles are your job. You have the tanks, with 3 choices from there. You want to be a different style tank their your PLD friend? Play WAR or DRK.