In What If...?, in the finale, Uatu the Watcher is the only one concerned about the multi-universe when
Ultron with the Infinity Stones plans to conquer the multiverse.
Why is Uatu the only one concerned about or who wants to save the multiverse? Why don't other cosmic beings interfere at this point?
Related questions that come to my mind and that can lead to an answer:
Could it be that other cosmic beings thought Uatu will handle it? or Could it be that they don't know yet how big a threat Ultron is? Couldn't Uatu ask other cosmic beings for help? Could it be that he knew the Evil Doctor Strange is more powerful than many cosmic beings and wanted to devise a plan with him?
From the plot standpoint, it would get complicated to include many cosmic beings. However, there can be an in-universe answer also.
Edit: Adding some key information I've missed.
"I have seen everything that has ever happened. Ever will happen. Ever could happen." ―The Watcher
"Uatu and Strange went back to Strange's dimension, the Mirror Dimension, where Strange pointed out how Uatu knew how the battle would unfold before it started as well as Killmonger's betrayal. Uatu then thanked Strange for helping him and his sacrifice to keep watch over the pocket dimension." https://marvelcinematicuniverse.fandom.com/wiki/The_Watcher
It looks like The Watcher has seen all outcomes in all timelines. However, when Ultron breaks into Nexus of All Realities to fight with him, he says: "I don't understand. This should not be possible."
It looks like this is the only thing he didn't see but then based on what Strange said it is implied that he knew how the battle will unfold.
It is not part of MCU currently but there are supposed to be the Protectors of the Multiverse.
"The Multiverse was originally categorized and protected by Merlyn, who established the Captain Britain Corps."
"There are other beings and groups dealing with protecting the Multiverse, for example the Time Variance Authority, or the Exiles." https://marvel.fandom.com/wiki/Multiverse