D&D 5E Line or Cone, why aren't there any breath weapons with a Burst Area of Effect?

[Voadam]

It should be noted that at least in 3e, "acid" really meant any type of chemical damage. Also, the main way chlorine gas works as a weapon is reacting with moisture in your throat and lungs to form hydrochloric acid (which is why you get pretty good protection with "just" a gas mask, unlike many other poison gases which can be absorbed through the skin).

In 5e, I can't find anything about their breath being specifically chlorine, but instead just generic "poison" as part of a general theme of corruption.

4e it did poison damage too. It was just described as poison gas with no reference to chlorine.

4e MM page 79:

"They breathe clouds of poisonous gas."

Monster Vault page 69:

􀀫 Breath Weapon (poison) ✦ Recharge 􀀞 􀀟
Attack: Close blast 5 (enemies in the blast); +8 vs. Fortitude
Hit: 2d10 + 3 poison damage, and the target is slowed and takes ongoing 5 poison damage (save ends both).
Aftereffect: The target is slowed (save ends).

4e in fact went deeper in on the non-chlorine poison aspect and in the Draconomicon had this sidebar for them:

"Some green dragons regularly dose the thorns and brambles of plants in their territory with their poisonous breath, eventually building up a dangerous coating. Anyone pushed or pulled into a thorn bush or briar so treated takes 3 poison damage and ongoing 3 poison damage (save ends), in addition to any damage the thorns normally deal. Manipulating or attempting to climb plants treated in this way results in 1 point of poison damage and ongoing 1 poison damage (save ends)."