Because they designed Healers to be accessible to everyone, even the lowest common denominator. Do you really think Raiders, especially Hardcore raiders, enjoy this incredibly simplistic design? There's a reason Yoshida specifically mentioned they aren't going back to the Heavensward design. Guess which demographic has been asking for that? Hint: It ain't the casual playerbase who struggles to keep up buffs like Heavy Thrust and Straight Shot.
This is primarily due to the fact most jobs in this game are extremely rigid and feel awful to play with prolonged downtime regardless of your skill level. Likewise, most players, especially melee, don't like essentially sucking their thumbs because they're unable to actually press their buttons since the boss jumped. Keep in mind, range attacks broke your combo until Endwalker. Therefore, they were rarely worth pressing. Put simply, it isn't fun watching a boss do pretty animations instead of you killing them... while doing pretty animations. That being said, we still have plenty of downtime fights. In Shadowbringers alone, Titania, Hades, Ruby, Emerald and Varos all have varying amounts of forced disengagement or straight going untargetable.The same can be said about boss mechanics, DPS uptime is now the most considered thing while designing mechanics. Bosses like what we had in Heavensward and A Realm Reborn where they go untargetable, either due to mechanics or the boss literally being invincible while they do a thing, also became extremely rare.
Casuals didn't complain that Leviathan would hide in water, or that they had to control the Gobwalker or become a Gorilla to push out bombs, or get jailed by Quickthinx...
It's not a coincidence that all of this changed one expansion after FFLogs became a thing.
Casuals did complain, incessantly, able Leviathan. Nobody liked the Gobwalker because it meant you basically weren't playing your job. Which, again, isn't fun. I can tell you right now how mechanics like that will be handled both in Normal and Savage equivalents. Everyone makes a healer, tank or the Prange do it depending on whether other mechanics necessitate the former two's responsibility. Why? Because you either don't need two healers/tanks for mechanics or you do, thus you naturally opt for the weakest DPS. Guess who that always is? Casual players are also why fights as a whole are considerably easier than they once were. Hardcore players weren't upset over Thordan and Sephirot's difficulty. In fact, many raiders even at the midcore level still lament how challenging those fights were. It was the more casual centric players who quite literally couldn't clear them. And the demands for nerfs were numerous. There's been threads each expansion that the Normal modes are too difficult.
Because it invalidated that content. What's the point of clearing the hardest content in the game if someone spamming Expert Roulette can get equal or even better gear? Case in point, Tome pants are BiS for tanks. So I have literally no reason to even look at P3S as a tank main were it not for Savage having exclusive upgrades until the follow up patch. You mention Diadem but ignore that a sizable portion of complaints came from casual players whose relics were also outclassed by those same Diadem weapons a mere nine days after the final step released. Most raiders didn't actually care all that much since the raid tier was long over. Several people simply criticised how desperate it made SE look they threw a RNG BiS weapon in content that was received so poorly, they've redesigned it three times.Gearing too is dictated by hardcores. It was hardcores that complained about every little thing that the dev team made for casuals. I remember very well the drama over Mythology gear in ARR being the same ilvl as Coil, which prompted them to extend the grind for casuals by nerfing tome gear and adding upgrade items exclusive to Coil. The Diadem that could reward BiS if you were lucky, also bad because potentially better than Gordias. Oh, and what about the relic grind... I've read so many predictions on relic being like "it won't be better than (x raid)". When the old Relic + 1 (later renamed to Zenith I think) was potentially better than Allagan weapons, until they bumped Allagan because... well, guess who complained.
If that were the case, we wouldn't have the current job design. Healers have been screaming for years to increase outgoing damage; tanks have wanted more responsibility like boss positioning, and DPS want complexity in their rotation. All of these are requests and criticisms from the "high end raiders." Considering Hydaelyn and Zodiark can be killed without even bringing a healer on the day of their release. Guess which demographic the dev team catered towards?Most of this game's battle content is designed around high end raiders first. I'm not saying that this is a bad thing, I really don't care, but I really dislike seeing how casuals are blamed for all of this game's shortcomings after seeing so many things become worse because they pandered to hardcores... hell I've seen them release a DoA extreme just because they were afraid of displeasing raiders and then rushing to fix him because nobody was doing him.