Why no one tanks...

[YuriRamona]

Based on your criteria, healers have much more responsibilities than Tanks.
There is ofc few fights entirely up to Tanks such as Garuda EX for example but in general ?
They dnt really need to care bout anything else than WHAT theyre targeting and WHERE.
Healers in the other hand have to:
Watch theyre positioning for optimal Aoe heals or dodging purposes, watch for enemy moves, time their casts, watch over everyone's health, watch their MP.

Ifrit EX is the best example that comes to my mind.

PS: Its coming from someone who can play each of the 3 roles in endgame content ( WHM main ).

Titan EX: Tanks must switch at fairly precise windows. Offtank must grab mini-Titans and secure enmity within 5 seconds. Heaven forbid you get hit by those landslides with a sliver of safe zones to stand on too.

Ifrit EX: Tanks must switch at very precise windows. Must be in front but not get hit by Incinerate. Must not be hit by Howling Wind while on the side. Must be on the edge when offtanking, dodging Howling Wind, dodging melee, dodging Incinerate, to place offtank Eruption appropriately.

T5: Tank positioning of neurolinks. Calls timings of phase changes (holding DPS). Decides when or when not to jump inside Conflagrations. Places snakes perfectly in position for LB while dodging debuffs.

Healer: Dodge stuff (same that DPS do). Cure constantly (i.e. take an action every GCD; which everyone does). Manage MP. Real difficult.

There's a reason I prefer to play SCH or WHM even though it is mind-crushingly boring.

It's because it's mind-crushingly boring.