Without a question, All For One and One For All are the two most powerful Quirks portrayed in My Hero Academia thus far. Close after is the Overhaul Quirk, which is held by Kai Chisaki, the previous Shie Hassaikai's youthful boss and a key enemy in Season 4 of the anime.
Because it lets its wielder to disassemble and reassemble matter into whatever shape they like, Overhaul has extremely versatile capabilities for both devastation and healing. So why didn't All For One, who has spent the majority of My Hero Academia dealing with his shattered body, try to take Overhaul for himself? He definitely recognized Chisaki after his escape from Tartarus, therefore it seemed likely that he would try to steal the Quirk during the raid - or possibly sooner.
All for One Was Unaware of Overhaul's Quirk
The most obvious explanation for why All For One wouldn't target a Quirk as powerful as Overhaul's is that he simply wasn't aware of its existence. As shown in My Hero Academia Chapter 312, the mighty villain recognizes who Overhaul is to some degree. After his escape from Tartarus, All For One meets with Lady Nagant and, noticing that she's accompanied by Kai Chisaki, acknowledges him as the young Shie Hassaikai boss. All For One then asks if he's a friend of Nagant's, and she replies that she's keeping him around as he might prove useful later.
All For One exhibits knowledge about Chisaki and his function as the yakuza group's leader based on this brief conversation. However, his knowledge of Overhaul may be limited to a cursory awareness of one of the criminal underworld's rising criminals. During his time in seclusion, All For One kept tabs on a variety of people of interest, both heroes and criminals. While Overhaul may have made the list due to his notoriety and standing as a crime boss, his Quirk may not have been widely known, leaving All For One in the dark about how beneficial it could have been to him.
The Overhaul Quirk of MHA Is Too Complicated to Learn
Another potential reason why All For One made no attempts to steal Overhaul is that the Quirk's learning curve was deemed too difficult. The villain has already stated that he avoids taking specialized Quirks that require extensive training to properly utilize. He would never target a power like Mirio Togata's Permeation, for instance; while it's one of the most useful defensive Quirks in My Hero Academia, it took years of training and concentration to master.
While Chisaki's Quirk's exact specs were never revealed, he possessed a thorough understanding of both biology and chemistry, which he used to create Quirk-deletion bullets from Eri's body. Because Overhaul allows Chisaki to change matter at the molecular level, a strong background in chemistry and biology is likely required before the Quirk can be used. In his opinion, the amount of time and research required to learn Overhaul for the intricate bioengineering that All For One would most likely entail, such as a reconstruction of his entire body, was not worth the trouble.
Furthermore, Overhaul is a Quirk that, much like Eri's Rewind, could have deadly consequences if mishandled. If All For One decided to steal it and use it to heal his body, it's entirely possible that he would blow himself up or alter himself so drastically that he'd be unable to function. It may not even be possible to use Overhaul to heal injuries as extensive as All For One's, as no other healing Quirk has been able to restore him completely. With such risks at hand along with the Quirk's high barrier of entry, All For One may have decided it wasn't a good fit and set his sights on other plans.
The Overhaul Was Not in Everyone's Plans
The final in-universe explanation for why All For One never showed interest in stealing Overhaul's Quirk is that by the time he learned about it, he was already too invested in his current plan: raising and eventually taking control of Tomura Shigaraki. His goal of transferring his mind into Shigaraki's body had already been in the works for over a decade by the time Overhaul became a prominent figure in the crime world. Shigaraki's Decay Quirk does overlap in much of the same destructive usage as Overhaul, making stealing it somewhat irrelevant for offensive purposes. Likewise, any defensive combat uses Overhaul has likely could be replicated with All For One's many other Quirks.
The largest benefit to stealing Overhaul would be for its healing abilities, but even this may not have fit All For One's overall desire for a newer, younger body. Putting aside the complications of learning how to use it, he may have just wanted to move on, having already invested time and resources into Shigaraki and his League of Villains. All For One has shown numerous times to have supreme arrogance and confidence in his plans. He may have even felt like taking Overhaul would make things too easy, an unnecessary side quest that would distract from his primary goals.
Of course, the most likely explanation for why All For One showed little interest in stealing Overhaul -- even when it was right in front of him -- is that My Hero Academia creator Kōhei Horikoshi simply didn't want to go that route in the story. All For One with Overhaul would be too overpowered, and would remove his only weakness and entire motivation for controlling Shigaraki. Maybe it would be more accurate to say his reasoning for showing little interest in Overhaul would be that it went against the writer's plans rather than his own. But while Overhaul and All For One's narrative paths don't seem to align, perhaps in the future the villain will give a clear, in-universe reason as to why he wouldn't try to steal one of the strongest Quirks in the entire series.
