IMO a big part why all this mess is happening is the fact that DD classes don't do much more damage than Tanks/Healers compared to other MMOs.
Its a point that some of you have mentioned.

For example

FFXIV: Tank does 1k, a DD does 1.4k. That is very close and the tank damage will be a big factor in the raid DPS. If the tank tweaks his gear and rotation to do 10% more (which is doable) he can even become much closer to lets say utility classes such as BRD.

Other MMO: Tank does 1k, DD does 4k. Here the bulk of the damage is from the DD classes, even if the tank goes out of his way and do 50% (which is HUGE in an MMO), he wont impact the over all raid dps.

Another issue is the fact that other MMOs empower the tank stance. Examples include access to abilities only usable in tank stance, those abilities define the tank role in raid/dungeon.

Another example is increase tank damage the more armor or vit he has (a ratio of course). This makes getting more beefy a double dip for tanks. More HP & armor = more damage. This wont make tanks do as much as DDs, but will encourage them to use tank items.