...for the three tank jobs, warriors had very high solo performance and were top choice as off-tanks. They had a lot of self-healing ability and burst power, so that job already felt close to complete for us.

But that's what's great about warrior. And since so many people use it, rather than trying to push warriors back, we instead focus on what puts warrior in the standing it's currently in, and supplemented the weaknesses of paladin and dark knight on that basis.
-Yoshi-P, Dengeki Interview, Stormblood
We see this idea thrown around a lot. If there's a power difference between two jobs, it's better to buff one than nerf the other. It seems nice on paper. Everyone likes buffs. Nobody likes nerfs.

Here's the problem:

Job buffs influence encounter design. Take mitigation for example. In ARR, your main tankbuster mitigation kit might have consisted of Rampart, Sentinel, and Hallowed. Because you had fewer cooldowns to work with, 20% DR was considered very good.

With each expansion, we've been given more cooldowns to work with. Partially to give players new toys, but also to keep parity between the tanks. The problem is, if you're given multiple cooldowns, raid design has to be adjusted accordingly. Damage values inflate, and you're expected to stack multiple cooldowns to survive. 20% DR generally isn't enough anymore. The difficulty with TBN, for example, is that it's an adjunct. You have to pair it with something else to reach a safe threshold. Damage inflation devalues the ability. In ARR encounter design, an ability like this would have carried a lot more weight.

The more damage values inflate, the more WAR's mitigation toolkit becomes preferable, due to the recasts of Vengeance and Holmgang. The more a fight encourages you to stack multiple cooldowns, the more powerful Holmgang becomes, since you can just bypass one set and give your other cooldowns time to recover.

The end result is a bit of an arms race. It's not enough to hand out a new ability to every tank. Giving out buffs changes the relative effectiveness of every existing ability in the toolkit. You actually have to re-evaluate everything from the ground up.

The same applies for raid defensive buffs as well. In a world where every tank only has access to Reprisal, all things are equal. If you start adding in more raid defensive buffs, the relative strength of Reprisal drops off, because you're expected to stack these other buffs on top. The solution to balancing PLD's raid buffs was never to turn Shake it off into Divine Veil Mk.2. The solution was to re-balance PLD's defensive buffs.

There eventually comes a point where job balance requires that you hand out nerfs, or else the whole thing becomes unmanageable. Also, the fact that "so many people use" a given job shouldn't play into whether you nerf it or not. Job popularity is a reflection of how easy it is to play and the rewards you get for playing it. With such a balancing mentality, it's easy to see why overpowered jobs remain unchecked. If you're afraid of the consequences of nerfing any given job, you haven't a hope of achieving balance.