The main reason I have heard for the existence of lavabending is this:
"Well, waterbenders can change the state of matter. why not earthbenders?"
To that I must say "Well, firebenders can create their element, why not everyone else?".
the bending disciplines are different. they are not parallel.
It was established in ATLA that firebenders are the only ones who can create their element. it was also shown that waterbenders are the only ones who can change the state of matter with their element. Why is it like this? Because it is. This was the premise. This is how it was written. and it was written this way to make the elements balanced, so nothing would be OP or UP.
If firebenders couldn't create their element they would be really UP. It is rare for fire to just be sitting around. so they would hardly ever be able to bend. There is plenty of the other elements in the world. This balances firebending with the other disciplines.
Waterbending was written to have more perks because it really is the least convenient. despite most of the planet being covered in water, humans live on land. also, ice and steam are not OP. getting hit by a chunk of ice is rarely fatal unless it is made to be sharp. also, no form of water wont hurt you to the touch.
so on top of the fact that lavabending has no excuse for its existence, lavabending is deadly and OP. a fist sized chunk of lava will hit you and you will probably die. how is this different than fire? fire isn't solid. fire will hit you and probably go out (unless you are covered in something flammable). fire will hit a lot of things and go out. yes it will injure you and could kill you. but, its no where near as deadly as lava. lava will hit you and keep burning and then drip down your body burning it. also, a lavabender can just keep making and making lava everywhere that you will have to avoid. lava flows around and hurts things by itself. yes fire can do that but, it is WAY easier to put out. even if you do cool some lava the bender can just change it right back. the firebender cant just make something catch fire, they must shoot fire at it. lavabending is practically the combination of earth, water, and firebending into one type of bending.
so do you see how OP lavabending is? I shouldn't even have to explain it.
I know it existed in some form in atla. but, only the avatar could do it AND MOST IMPORTANTLY, even the avatar couldn't turn solid earth into lava, he/she could only bend existing lava (which is pretty rare).
So reasons in a nutshell 1. No excuse for it's existence. 2. most OP bending behind anytime bloodbending (which is also stupid as hell).