The Avengers are known as the most famous superhero team, playing a crucial role in the Marvel Cinematic Universe (MCU). They often participate in significant events that directly impact the fate of the world and, in some cases, the universe itself. However, there have been instances when this formidable team has been unable to unite and fight together. Numerous horrifying tragedies have occurred within the MCU without the presence of the Avengers to aid them.
The Convergence Event

The bomb from Thor: The Dark World first introduced the concept of dimensional rifts on the big screen, simultaneously bringing about a battle on Asgard during the Convergence event. This phenomenon has the potential to distort reality and merge different dimensions into a single suspended state, which could be completely annihilated.
However, only one Avenger participated in this event, namely Thor. Even when the battle reached Earth, no other Avenger appeared to assist the God of Thunder. After all, Thor, along with his own strength, was able to defeat Malekith to prevent the destruction of the Universe, including Earth. The Convergence event is rarely mentioned in later MCU projects.
The Fall of S.H.I.E.L.D.

The bomb from Captain America: The Winter Soldier revealed a massive infiltration by H.Y.D.R.A into S.H.I.E.L.D, the organization behind the Avengers’ operations. This event led to a significant battle in Washington D.C., where H.Y.D.R.A launched three aircraft carriers to activate Project Insight.
During the battle, a large part of Washington D.C. was devastated, and there were still repercussions that the Avengers would face after every superhero encounter. However, only Steve Rogers and Natasha Romanoff were the two members of the Avengers who participated in this event. Despite being two crucial members of the Avengers, they could not contact or receive help from other operatives, making the situation even more severe, with many casualties.
The Return of the Goddess Hela

In the storyline of Thor: Ragnarok, Marvel Studios introduced another offspring of Odin, besides Thor. This character is Hela, the goddess of death, who was imprisoned in the underworld of Hel for seeking to conquer the Universe. After being freed, Hela quickly set out to conquer Asgard, declaring herself queen and confronting Thor with Loki on another planet.
The chaos that Hela could unleash would have had severe consequences in the Universe, including Earth. While Thor faced numerous difficulties in his battles against Hela, he could not call for assistance from other Avengers on Earth—those who could have helped him protect Asgard. Broadly speaking, Thor often only collaborated with those he casually encountered during his journey, in this case, Hulk and Valkyrie.
Mysterio – The “hero” bringing a “different reality”

Spider-Man: Far From Home showcased one of Peter Parker’s most significant challenges—Quentin Beck/Mysterio, on the big screen. Relying on drone technology that Tony Stark left behind, Quentin was able to manipulate the events into superhero-like attacks to approach Peter.
However, the illusions created by Quentin caused real destruction, particularly during the intense battle in London in the film’s final act. Despite being labeled as “the newest Avengers recruit,” there was only one Avenger present at the time (and he ultimately suffered defeat), which was Spider-Man. Even when a major city was under siege from thousands of Stark-designed drones, the Avengers were nowhere to be seen.
The Goddess Ammit

The climax of Moon Knight brought about an intense battle between two Egyptian gods – Khonshu and Ammit in Cairo, causing a significant upheaval. However, the Avengers remained unaware of this event, and the battle was never mentioned in subsequent MCU projects after Moon Knight.
Looking at it more broadly, the series Moon Knight has minimal connections to the overarching MCU storyline, and there are practically no familiar characters contributing to this project. After the traumatic experiences caused by Thanos, it seems hard to believe that the Avengers would remain oblivious to the terrifying battle of the two Egyptian gods occurring on Earth.
Dormammu

The battle between Doctor Strange and Dormammu can be seen as one of the most iconic and absurd moments in the MCU, where both are trapped in an infinite time loop, unable to escape the manipulation of the time stone that Strange possesses. However, just a few moments before that, the audience witnessed the overwhelming power of Dormammu, who had previously destroyed Hong Kong before Doctor Strange could intervene.
Yet, despite the fact that a formidable villain was wreaking havoc in another dimension while still on Earth, no Avengers member appeared. Considering the timeline, the Avengers were embroiled in a conflict against their own teammates during the events of Captain America: Civil War.
Ego

Ego is the father of Star-Lord Peter Quill and Mantis in the MCU, while also being a significant character in the blockbuster Guardians of the Galaxy Vol. 3. He is a Celestial with ambitions to create a universe of his own based on his own designs. Ego spent many years traveling through various planets, including Earth, to sow his seeds and prepare for his grand plan. However, the Avengers remained unaware of Ego’s existence or his ulterior motives.
The Appearance of the Celestials

One of the main storylines of Eternals revolves around the threat posed by the Celestial Tiamut – a process that could lead to Earth’s complete destruction. To protect humanity, some members of the Eternals decided to intervene and stop Tiamut’s awakening, resulting in the Celestial’s total demise while it was still emerging from beneath the surface of the Earth.
Yet the Avengers remained oblivious to this event, despite the fact that a part of Tiamut’s massive body had drawn significant media attention in the MCU. Additionally, when Arishem decided to appear in the sky above Earth during the final act of Eternals, the Avengers still did not take any action related to this event.
The Transformation of Wanda Maximoff/Scarlet Witch

Doctor Strange in the Multiverse of Madness revealed that Scarlet Witch is one of the most powerful Avengers in the current timeline. Her mastery of Chaos Magic, coupled with the Darkhold magic book, has allowed her to wreak havoc in Kamar-Taj, under the watch of many powerful sorcerers, including Wong and Stephen Strange.
The Avengers did not investigate the existence of the threats in Westview caused by Wanda Maximoff. However, even when she completely transformed and sought to utilize the multiverse to bring back her two children, the Avengers still did not intervene. Even in other projects, they also did not mention what Wanda experienced and the trauma she endured after the losses in Infinity War and Endgame.
The Dark Variants of Doctor Strange

There are rare instances where the Avengers have been caught off guard by the dark realities stemming from the Marvel multiverse. Many timelines have introduced dark variants of Doctor Strange, such as Strange Supreme in the animated series What If…? or Sinister Strange in Doctor Strange in the Multiverse of Madness.
In both instances, the Avengers were completely unaware or had no means to stop the occurrences that were happening in their own realities. For instance, Doctor Strange of Earth-616, the main timeline of the MCU, utilized the Darkhold magic book, the Avengers should have been cautious and more attentive to these superheroes’ actions.
Source: Marvel Studios, ScreenRant