I agree with this for the items that actually have a character animation tied to them, best example off the top of my head being Realm Reborn Red. The fireworks are mostly (all?) just a graphical effect that originates from the ground in front of your character or from your character's hand while they do nothing.

We have so many different fireworks that are just recolors or .jpg swaps that they would need to make a whole Fireworks category if they were to turn them into emotes. But also let's face it they won't get rid of their best source of consolation prizes for the various lootboxes in different content.