Because breath attacks are generally inefficient against many high level enemies anyways?

The Dragoon wyvern does low damage against high level enemies. This is nothing new. Breath resistance is far, far, far from the main reason for this. Any enemy with vaguely decent magic evasion will consistently resist most of the breath damage. It also has very little impact on the Dragoon's total damage output. Under ideal conditions, wyvern breath attacks never make up a huge part of the Dragoon's total damage because wyvern auto-attack damage is more significant in any condition short of spamming weapon skills so fast that the wyvern never auto-attacks.

Breath attacks are never the best damage spells in the Blue Mage arsenal. The only reason anyone should ever build around them is against enemies where their defenses and resistances are exactly setup in such a way that they are the best damage spells in their arsenal. If they aren't, then they should be using something else. Under ideal conditions, Heat Breath and Thunder Breath both have low potential damage, a high MP costs, long casting times, and long recast times. Under normal conditions, they do pitiful damage, but have a ludicrously long casting time, a ridiculous MP cost, and, to add insult to injury, an extremely long recast time. Most of the better damage spells could be more mostly resisted and still outdamage every breath spell in the Blue Mage arsenal.