The first season of Marvel Studios' What If...? saw Clark Gregg return to the now-iconic role of Agent Phil Coulson. As a result of his appearance in a Marvel Cinematic Universe show, it's hard not to ask why he couldn't have reappeared in Avengers: Endgame. After all, James D'Arcy's Jarvis from Agent Carter appeared when Steve and Tony traveled to the 1970s, making him the only Marvel Television character so far to cross over into the MCU. Upon further examination, however, the lack of Coulson in Endgame comes from multiple variables surrounding the circumstances of his place in the larger MCU.
During the first phase of the MCU, Coulson played the role of the audience and was the middle man between almost every solo film leading up to 2012's The Avengers. In that time, he was revealed to be a stern yet humorous believer in Nick Fury's plan to unite a group of remarkable people. He was also an optimist and a big fan of Captain America. Sadly, he was killed by Loki, becoming the catalyst for the Avengers to form. He wasn't seen again in the films but was resurrected and starred in the series Agents of S.H.I.E.L.D. for seven seasons.
From a behind-the-scenes perspective, Coulson's appearance would have left a handful of fans confused if they hadn't watched Agents of S.H.I.E.L.D. Over the show's many seasons, Coulson was resurrected and ultimately killed off again before returning as a Life Model Decoy for the final season. As a result of his robotic fate, he looks no different than when he did in the first film. Not only would his appearance have bewildered many, but his lack of physical change may have only added to the confusion.
From an in-universe point of view, Coulson's death is one of the most important events in the timeline. Without his sacrifice, the Avengers might never have formed, and Loki might have succeeded in taking over the planet. That being said, Coulson's death isn't mentioned past the first film, so it's hard to say how much of an impact he still had on them years later. While it would have been over a decade since his death, his return during a scene like Stark's funeral, where many fans wished he had appeared, would've raised more questions than answers. It's also important to note that with the five-year time skip, Agents of S.H.I.E.L.D. never caught up to the events of Endgame, so Coulson's return would've created a gap of lost time that would've needed to be clarified.
Coulson's return also adds more time to the film's three-hour runtime. Plus, with the questions the original Avengers would have had, this could've required a new plotline to explain events to the team and the audience. But had this happened, the TV series' glaring question would've also been answered: is Agents of S.H.I.E.L.D. canon? As of now, the answer is still murky since there hasn't been enough evidence to definitively prove it is or isn't. If it's not, then Coulson's absence is easily understood. However, if the show becomes canon, the aforementioned questions would take precedence the moment he reappears.
With the multiverse playing an even more significant role in the MCU, it could only be a matter of time before the characters and events of Agents of S.H.I.E.L.D. come back in full force. But for now, that is nothing more than a dream held by many fans of the show and characters like Coulson. These fans felt his absence in Avengers: Endgame, and his return in What If...? is more than enough to reignite hope for his return. Until then, fans can only wait and put the pieces of a new multiverse together in the hopes that it will lead to Coulson's return.