Guardians of the Galaxy director James Gunn has frequently said that Adam Warlock is being saved for Vol. 3 (something which one of Vol. 2's after-credits scenes confirmed) and Joe and Anthony Russo have now confirmed that. While the movie's screenwriters revealed that the hero was included in an early draft, they've made it clear that they never had a plan in place for him to help out the team.
"That’s a James Gunn thing — we never intended to put Adam Warlock in Infinity War," Anthony says. "We have so many heroes we have to service that you guys love and give them screen-time." He went on to admit that the only way to include Adam would have been to essentially force him into the story, something which would have meant he had no real emotional connection to the Mad Titan.
"You want to see the hero and villain to have an emotional connection," he continued. "You wouldn’t have a connection to him [Warlock] as he doesn’t have a connection with Thanos and a built-up story." That makes perfect sense and as cool as it would have been to see the comic books recreated with Adam taking on the villain, his introduction is best saved for Guardians of the Galaxy Vol. 3.