When MCU teased Adam in the end of GOTG vol 2, I was sure that he will play an important role to defeat Thanos but haven't heard of that guy since then. I thought I would see him the Endgame but they didn't show him.
Because that's not the story Marvel wanted to tell.
Introducing Warlock (a new character) was unnecessary and would have blunted the main thrust of Endgame. It's likely we'll see him in Guardians of the Galaxy Vol. 3.
James Gunn tweeted that he was never going to be in Endgame.
Said Gunn: “Adam Warlock will NOT be in Avengers 4 (and I’ve never said he will be in Vol. 3).” Gunn also said that Guardians of the Galaxy Vol. 3 would take place after Avengers: Endgame, then still known as just “Avengers 4” by fans.