Technically they are.
Fundamentally "roles" break into 2 primary gameplay designs.
(1) Offensive (win condition)
(2) Defensive (to make it feel less of a numbers game, and add tension in a tug of war)
Further divided by sub roles;
(A) Selfish (high self sustain usually)
(B) Support (good synergy with the team, aiding in numbers/effectiveness)
Lastly, role/sub roles are divided by aesthetics, flavor, and gameplay mechanics; (Strengths and weaknesses apply)
Ranged, at the cost of Cast bars/mobility.
Melee, at the cost of positionals/mobility.
(There are more, such as pets, burst vs sustained, faster paced GCDs, etc)
NIN is a support with positionals, so its odd BRD and MCH dont have equivalents for thier mobility/freedom.
Though one could argue their lack of gap closers and anti pushback, which I'll agree comes pretty close, so its ok. (plus part of their flavor.)
Now, back to the point, tanks are "supposed" to be under the defensive/selfish category.
They are the equivalent of a Selfish Healer, who can only heal themselves (rampart/potent self mitigation).
They lost decent direct healing (while healers lost decent party targeting mitigation) for the sake of doing far more DPS.
Most people see tanks as melee DPS with slight/occasional mitigation.
(Here's where it gets complicaited.)
The reason they arent an Offensive role, isnt because of their mitigation despite being a big reason, but due to how other DPS dont actually raise the tanks DPS to the same levels of other DPS, and due to healers not having propper mitigation tools. For tanks to be primarily an offensive role, they need to be a DPS with enmity control, and no/little mitigation. (other DPS and Healers make up the differences)
They need to do the same damage as the other DPS at the end of the day (or roughly equivalent). While non tanks are the types to support the tanks lack of personal DPS. (Think DNC.) So that tanks are raised to the same DPS level of the other DPS.
Now healers need to have heals better suited to a single target, and heals better suited for raid wide dmg. The person who would get the most use of a strong single target heal (something like aldo, but with a shorter shield timer, so u want to keep it on a single target) is the person who gets hurt more often.
Pretend healers can put a "tank stance" on one person, but cant reapply it to another person for 30seconds. This is why u want a DPS who keeps the focus of the enemy.
TL;DR tanks are currently (in most MMOs) designed as a jack of all trades. They arent fully the defensive role, nor the offensive role. This is because old tanks were more so designed with the defensive role in mind, and to make tanks more appealing to more players, they started trying to make them the "DPS with mitigation" which is technically in the middle, and not an actual DPS. (as explained above)
Being a hybrid melee healer/DPS doesnt mean you're equal to a pure melee DPS.
If you equate them as being the same, apply the same logic to healers as casters, and all ranged as the same function, and melee/ranged/caster DPS as the same role. (consistency)
I agree with this.
If I were to try and redo the classes from the ground up, I'd be hard pressed to give a butt about Physical Ranged play styles. (and more accurately, DPS in general)
But when it comes to tanks and healers, I have tons of interest, with losts of ideas, and also an understanding of which ideas cant work with the other ideas, w/o changing the core of how I wish the game to flow.
One example here, is how they did intend healers to cast DPS spells, but also didnt take that DPS into consideration. The only way to allow GCD DPS, is to have bosses do very little fluff dmg between Tank Busters. But in order to make that considered "difficult" basic healing needs to be very weak, so you would be required to spend multiple GCDs getting them back up after a TB. They made oGCDs mostly for mobility. But here's the problem... You can use oGCDs to replace direct GCD healing. If you do that, what gets interrupted isnt always healing, but actually your DPS. Only in 4man leveling dungeons would tanks take consistent damage, to make up the lack of this dynamic. This is an example of them not understanding the finner elements of defensive roles.
I can easily see all of this, but when it comes to the DPS, i'm much more inclined to make similar mistakes, due to just not caring about the DPS roles as much. I really just dont care about the DPS role, unless they get some more party responsibility. (which then I'd start to care more.)