That said, Christopher Markus makes a clear point by addressing the fact that Infinity War was already too densely populated to bring someone as complicated as Adam Warlock into the equation.
In reference to the Marvel source material, including Adam Warlock in the events of Avengers: Infinity War arguably would’ve made plenty of sense in the long run.
With no solo movie to get him set up before this crossover, they instead opted to leave him on the table.
After all, the character is a fundamental element of the original Infinity Gauntlet story (one could even argue that he’s the main character of that book), so his presence in an adaptation of that story would seem like a logical step.