Because the whole purpose of selfish jobs is their "utility" is raw damage. If another job can do comparable numbers but offer raid buffs, a personal shield and range flexibility, it becomes objectively better at pretty much everything.
Keep in mind, cDPS is the better metric to use when comparing jobs against one another per FFlogs own description. For reference sake, cDPS is essentially aDPS + damage you received from other players excluding single target padding (Dance Partner and Astro cards)
Using this metric, Picto absolutely dominates every single fight in the game thus far—pulling noticeably ahead of both Samurai and Viper in all but two fights where it either just barely trails the former or is roughly tied with both. That's simply way too strong for a job with both raid utility and range flexibility. If Black Mage were brought up to this level, you'd equally have to buff both Red Mage and Summoner otherwise even the raise wouldn't be worth taking either. This, in turn, causes issues with the Prange who are already far too weak as it is. There's serious discussion double Melee/Caster (assuming BLM buff) would be superior despite a full loss of 1%. So they now need to be buffed.cDPS allows you compare how well you and your raid played into burst windows and AOE buffs. It's a good way to look at job balance between each other, and seeing which jobs have strong buffs while also providing good damage during other job's AOE buff windows.
In other words, SE either needs to buff six jobs or slightly nerf one. And if they do go for the former, it means in any potential downtime heavy fight like what M3 looks to be, double melee is objectively worse in every way. That isn't to say I don't think double Caster should be a viable option. It should but there's a difference between being viable and just outright better. Jobs like Dragoon, Ninja and even Reaper couldn't compete with a buffed Black Mage to Picto levels.