According to the comics, the Hulk has very fast regeneration abilities similar to Deadpool.
At the end of Avengers: Endgame, the Hulk has his charred right arm in a cast because of the burns from using the Infinity Gauntlet to "unsnap" everyone who died at the end of Avengers: Infinity War.
Why hasn't it regenerated back yet, or even partially regenerated?
Because that's not an ability that they've covered in the MCU. In the movies so far they've had The Hulk be basically invulnerable, in fact getting stronger as he soaks up hits in his standalone films, we haven't seen him do any healing at all on screen let alone at an accelerated rate. I understand that Endgame is already a long movie, ten more minutes run time to explain that The Hulk can regenerate, and oh that's always been a thing we just hadn't noticed was probably not a good idea.
Alternately the fact that the damage was done by the Infinity Stones may mean that he simply can't regenerate.
Joe Russo has confirmed that the damage is "permanent" and compares it to the damage Thanos receives from using the Infinity Gauntlet. As such it is safe to assume the reason is because the damage came from the Infinity Gauntlet and so the Infinity Stones. The amount of power needed to unsnap, and in Thanos' case snap, is what caused such irreversible damage.
“He’s lost an arm,” Russo says. “He lost Natasha. That’s not coming back. He’s damaged himself. I don’t know. It’s interesting. That’s permanent damage, the same way that it was permanent damage with Thanos. It’s irreversible damage. His arm, if you noticed, is a lot skinnier. It’s blackened. So, he loses a lot of strength there.”
Russo does acknowledge that “permanent” means something different a world of magic and super science, but that’s not his concern any longer.
“But who knows? There’s a lot of smart people left,” he says. “Maybe someone helps him repair that. Maybe someone gives him a new arm. I have no idea where that character goes from here. The nice thing is we didn’t have to pay attention to where it goes after this, we just try to tell a satisfying ending.”
Comicbook, Avengers: Endgame Director Reveals Hulk's Injury Is Permanent
In the comics Professor Hulk has the benefit of Banner's brain but is no where near as strong as primitive Hulk. In fact when Professor Hulk gets to angry he reverts to “puny Banner” with Hulk's primitive mind.
Therefore this delay in healing does follow the comic book lore.
In addition this is an Infinity Gauntlet. Banner already states that it gives off tremendous radiation that will hurt him.
Using the gauntlet straight up killed Tony. Hulk survived with merely a burnt arm.
That is already a huge testimony to his regenerative powers!
Banner said in the The Avengers he shot himself in the head and Hulk spat the round out... seems like pretty good/quick healing to me so they just messed up.
Banner: In case you needed to kill me, but you can't! I know! I tried! I got low. I didn't see an end, so I put a bullet in my mouth and the other guy spit it out!
The Avengers
During the secret war ark, they had Banner Hulk - which is effectively Bruce Banners brain in Hulks body. Something we seem to be seeing in endgame. Professor Hulk is a combination of Savage hulks strength, Banners mind, and Grey Hulks cunning. Not the same thing. On top of this, Prof Hulk is actually consider the strongest hulk at a baseline level (disregarding the madder/stronger factor, which savage Hulk can utilize to his advantage due to being... well, savage). Or was, up until World war hulk, which seems to be simply the grey/savage combined.
Its all very confusing, but there are many Hulk's running around in the comics.One cannot simply assume something for the film; based on what they understand from the comics, because its not the same thing. Hulk heals because Prof Hulk has regenerated 90% of his body mass in comics doesn't quite work, because we don't even know what Hulk this is, let alone what his capabilities are. That being said, hopefully the arm Heals, and Hulk gets some actual fight time his next go around.
