WARNING: The following contains spoilers for Eternals.
Eternals certainly had an ambitious task ahead of it, entering into the Marvel Cinematic Universe after it is already thoroughly established while introducing a team of new characters to serve as the film's main super group. With millennia of lore attached to each of them, different power sets and an ambitious plot that threatened the entire solar system, there was certainly a lot to keep juggling for the film's story.
As the MCU presses on into the future and integrates the Eternals further into that world alongside other longstanding characters, it may be hard to make enough space in one story for both the Avengers and the Eternals; unless, of course, the Eternals were the villains. Set up as an in-fighting family who do not all necessarily have humanity's best interests at heart, the Eternals may not turn out to be the heroes they were set up to be in their debut film.
Eternals set up a bait and switch when it came to the moral paradigm of the story. The titular team were introduced to viewers as cosmic representatives tasked with protecting the world from the monstrous Deviants who threatened humanity. With their most powerful member, Ikaris, leading the charge, the super team fought off the Deviants through much of history until their threat was seemingly gone, but the central twist was the Eternals were actually protecting the Earth so it could incubate a new Celestial, whose birth would destroy the solar system, and with Ikaris embracing that goal, the team was torn apart as those closest to Ikaris were forced to challenge him in order to save the world.
It was not the first time that conflict drew the team apart, either. The mind-controller Druig splintered from the team when the Eternals' mandate to stay out of the politics of humanity became too much to bear, and the inventor Phastos retired from adventuring altogether when his technological advancements led to the atom bomb that devastated Hiroshima. More like a family than a super team, the Eternals represented a range of beliefs and opinions that often put them at odds with one another as much as it brought them together. If they can barely get along with each other, just imagine how impossible a task it will be fore them to ally effectively with the Avengers.
Marvel Comics recently began teasing an oncoming war between the Avengers, X-Men and Eternals that could easily be a setup for bringing that conflict into the world of film. While many of the Eternals seem to care for the best interests of humanity, and their new leadership under Sersi shifta them markedly in that direction, if the film proved anything it's that moral conundrums are too complicated and multifaceted for "the good guys" to be one united front. By building up the conflict between the teams in the comics, it's actually astounding just how well an Eternals vs. Avengers set up could work.
As previously mentioned, the MCU is bursting with characters, and every time there are new casts or threats to establish, it strains the narrative to juggle everyone's stories at once. Any given Avengers film, and indeed Eternals itself, already had such large teams that giving each member the space they deserve is almost impossible.
By pitting the teams against each other, there is extra space that would have gone to establishing a villain that can instead be afforded back to the characters audiences already know and love. However, the chief interest of an Avengers vs. Eternals movie isn't just in its narrative convenience, but in the appeal of just how much fun the core conflict would be to watch that boils down to one question: Who would win?
A psychic battle between the Scarlet Witch and Druig or a knock-down-drag-out between Ikaris and Captain Marvel just sounds like too much fun to ignore. Pitting the heroes of Marvel against one another just as often as against the actual villains is a tradition that runs back to the comics' earliest days, after all, as seen with Civil War.
The cosmic scale of the Eternals, with the film ending on Sersi, Phastos and Kingo pulled into space by Arishem to judge the fate of humanity, means they simply operate on a whole other level than the Avengers. Since they first came together to save New York City from aliens, the Avengers made their reputation on battling threats from space too large for any one of them to take on. The Eternals fit that mold perfectly, but they may just be too large a threat for even the Avengers to combat.