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.
scifi.stackexchange.com/users/87571/Ash
Because the damage was caused by the Infinity Gauntlet 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
scifi.stackexchange.com/users/58193/TheLethalCarrot
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.
scifi.stackexchange.com/users/86251/Richard C
MCU Hulk does not show deadpool levels of regeneration.
While he does have regenerative capabilities they are nowhere as fast.
There's a very significant amount of damage that was inflicted.
For reference: Using the gauntlet straight up killed Tony.
Hulk survived with merely a burnt arm.
That is already a huge testimony to his regenerative powers!
scifi.stackexchange.com/users/12551/Stark07
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
scifi.stackexchange.com/users/116342/Kumo
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?
In Endgame, why didn't the Hulk's arm regenerate?
scifi.stackexchange.com
<marvel><marvel-cinematic-universe><the-incredible-hulk><avengers-endgame>
scifi.stackexchange.com/questions/211165
scifi.stackexchange.com/users/115747/Kyza
There are multiple possible explanations for this, and the answer will be speculative. However, it is possible that the Hulk's regeneration capabilities haven't been fully detailed in the film, or even that they're not fast enough to regenerate quickly, or at least not as fast as the question states. It's also possible that the damage inflicted by the Infinity Stones is so great that even signficant regneration powers would take a long time to take effect. This is not explained but the main reason may be that the ability to regenerate to that extent is not covered earlier in the MCU and possibly the damage done by the Gauntlet is too great, with radiation etc.
Either because the ability to regenerate was not covered in the MCU or because the damage done by the Infinity Gauntlet meant that regeneration was impossible (because of the radiation). In the MCU Hulk does not have Deadpool levels of regeneration or at least not as fast, and the damage done by the Gauntlet is very extreme.