Avengers: Infinity War screenwriters Christopher Markus and Stephen McFeely have explained that including Adam Warlock in the movie would been “a massive backbend,” requiring a whole other movie just to introduce the character first.
“We didn’t introduce Adam Warlock, because it’s a massive backbend and you practically have to make an Adam Warlock movie to introduce him.
There has long been speculation about the character making an appearance in Avengers: Infinity War, given his strong connections to the Infinity Stones and the Infinity Gauntlet.
He notably led the fight against Thanos in The Infinity Gauntlet, the comic book upon which Infinity War is based.