Ehh... It's not always the case.
Since, there have been times in MMO's when the top tier tanks were the ones that were most self sufficient, which meant that the healers could spend more time DPSing, or they could replace a healer altogether with another DPS, replace the off-tank with another DPS (Because said self sufficient Tank could cheese through boss mechanics #JustDKThings) or otherwise adjust to push out more DPS than if they just took the Tank(s) that would provide the most personal damage.
Though this was alongside more options for healers, such as builds that would work via dealing damage to things acting like a 0.5 healer 0.5 DPS. Or other mechanics such as WoW's Shadowpriest giving an aura that gave lifesteal to everyone so basically everyone had a mini-Bloodbath active at all times. I suppose the FFXIV comparison would be running a RDM over a healer to primarily damage but also spot heal with Vercure and help out against deaths with Verraise. (If not also including any changes to classes, such as if Mantra was turned into a mini-Bloodbath effect for the team)
Heck, I remember some boss encounters in WoW where you just didn't need a healer at all because a Death Knight could cheese the bosses mechanics so hard that damage wasn't taken due to various things like AMS removing the stacking debuffs as well as percent health healing with a exponentially increasing health pool.
Whenever it's the case of Tanks doing too much, it is always, without fail, due to how much damage they push out.
Since, in order to try and make DD players want to play tank, they give Tanks massive damage.
Which leads directly to the issue of "Why bring a DD class when a Tank does the same damage but doesn't die?"
This is literally the issue that's being faced by Devs here too, they're pushing Tanks into this place where they don't have to care about anything other than stacking damage and pushing DPS combos and then they're surprised when Tanks are doing "Too much damage"
That is why it is necessary to divorce Tanks from this one-dimensional ideal wherein all they care about is stacking DPS stats and pushing Fell Cleave.
Be it by making mitigation stats useful, making Enmity/Mitigation combos a thing that are worth using, by creating more boss mechanics that need tanks to do something, by changing how bosses deal damage so it's not so heavily invested into specifically timed tankbusters, or a plethora of other things.
Tanks get into trouble for "Doing too much", when, ironically, they have little to do. When they don't care about maximizing Enmity, when they don't care about maximizing mitigation, when they don't care about positioning bosses or reacting to various mechanics, when they don't care about using any support skills to help or protect allies. When literally, the single thing they have to care about, is "Do More Damage"
Which is where the comment around Tanks becoming useless punching bags. If you take away everything but their damage, then people complain that Tanks are "Doing too much" even when they have only damage, so you take that away... What are Tanks left with?