Why exactly do Galvanize and E. Diagnosis overwrite each other?

[Sinstrel]

The jobs are so homogenized that its likely they literally just copy/pasted SCH's ability, renamed it, and gave it a new animation with some number changes

[WaxSw]

Balance.

Double shield stacking could allow for players to cheat the "dance" since many of the mechanic's debuffs are applied the moment they damage the player and a shield that covers the whole damage nullifies the application of those debuffs.

It also increases the problem of double shield comp being far superior healing wise than shield + regen or double regen

[Astronis]

Double shield stacking could allow for players to cheat the "dance" since many of the mechanic's debuffs are applied the moment they damage the player and a shield that covers the whole damage nullifies the application of those debuffs.

Virtually guaranteed it's this. If you can avoid the debuffs via shields, there's all manner of mechs you can cheat.

[Daeriion_Aeradiir]

Balance.

Double shield stacking could allow for players to cheat the "dance" since many of the mechanic's debuffs are applied the moment they damage the player and a shield that covers the whole damage nullifies the application of those debuffs.

It also increases the problem of double shield comp being far superior healing wise than shield + regen or double regen

Every single debuff in Savage+ these days always applies regardless of how much damage you take, they intentionally started doing this in EW when lots of vulns were being ignored in Eden tiers due to super shielding.
If any direct mechanic even remotely dangerous in any fight's "Dance" could be ignored with shielding, you 100% bet stacking tons of party mit on it to zero it out would become mandatory since SCH can save a crit-adlo for it, but that's not the case, all debuffs that matter get applied anyway.

That being said, it is the general correct thought process. The shields don't stack because most healing in higher end content comes from preventative measures rather than reactive healing, so double shields would be stupidly OP, especially in phases like TOP P6 that are entirely mit-check based rather than healing based. Double shield healer is already insanely strong even without stacking shields because of the extra mit, preventing shield stacking is basically the one thing that merits exploring AST/WHM as the other option.

[Rongway]

Double regen doesn't prevent anyone from getting one-shot by a big hit. Double shield can.

[Kohashi]

AST and WHM regens can both persist at the same time, and even non-core healer shields like Divine Benison and The Spire can exist on top of other shields, including Galvanize and E. Diagnosis. Why exactly do these two abilities overwrite each other?

This was even the case back before SGE, when AST sects were still a thing. Nocturnal Sect's Aspected Benefic overwrote Galvanize and vice versa.

Is there some kind of balance reason for this that I'm too stupid to understand? Because it just makes it feel bad whenever you queue into a PUG as a shield healer and find out that you're co-healer is also a shield healer.

Because HoT is garbage and divine benison/nocturnal sect is a limited resource.

Having 2 shields stack is incredibly broken in some content and it will encourage a certain type of play while excluding certain classes.

[noumen0nn]

It's a balance thing, like others have said. Shields are very powerful, hence why the majority of them are tied to long-cooldown abilities. Things like Divine Veil and Shake It Off can stack, because the merit of getting a super-powered shield is balanced by the cost of committing both resources at once and not having either available for the next 90 seconds.

If you could stack SCH and SGE's gcd shields, it would trivialize even the mitigation checks in ultimates, for the relatively low cost of one broil/dosis every time you do it.

Double shield stacking could allow for players to cheat the "dance" since many of the mechanic's debuffs are applied the moment they damage the player and a shield that covers the whole damage nullifies the application of those debuffs.

While this is true, there are more debuff-applying mechanics that don't work this way than ones that do, and the ones that do can already be cheesed by stacking other shields or using SCH's recitation-aldo-deploy. Still, to be fair, that also costs a resource, whereas with stackable gcd shields it wouldn't.

[DiaDeem]

Because you shouldn't run double Shields on Savage, you evil creative gamer!!1
Now go wall-to-wall HM ARR reworked Dungeons with our streamlined horrible Trust system.

[Kohashi]

Because you shouldn't run double Shields on Savage, you evil creative gamer!!1
Now go wall-to-wall HM ARR reworked Dungeons with our streamlined horrible Trust system.

pressing 1 or 2 buttons is now creativity, much wow, much amazement.

The funny thing is, double shielding in savage is actually a thing, however it requires some coordination.

[AmiableApkallu]

Fun thing I learned today: Differential Diagnosis and Catalyze do stack:



(Edit: Not the shields the OP was talking about, but it feels relevant, all the same. )