Tactics for the Swamp (Fortress).

Of course you are right... BUT wyverns still have too little health... Very little indeed! Although defense, of course, adds to their survivability...

The weakness of wyverns is balanced by the strength of gorgons. And such a decision is not unique to this race; one can recall, for example, naga-djinn.
Furthermore, wyverns can be obtained on the map (usually, however, on a random one) in "dragonfly nests" and their number can quickly reach large proportions.

Added after 3 minutes

But because of their speed, they still go into battle first - and accordingly, die first in quite large numbers.

Why necessarily first? Use dragonflies to remove retaliation + weaken them and then the wyverns move forward, after a wait. And if you also have teleport, then.....