Other than off-screen explanations, its notable how odd the TV side of the universe and the movie side of the universe interact very little between each other, at least nowadays. But one big plot point that the TV series and the movies share, is the concept of time travel.
In Avengers: Endgame, Tony Stark finally finds a way of time travelling, by using the quantum realm, almost five and a half years after the snap. It really does seem to be the Avengers only hope in going back in time and revert the snap.
However, one cannot feel indifferent to Runaways and Agents of SHIELD, both estabilishing two different forms of time travel, which is able to create alternate timelines on their own, or preserve old timelines. Why didn't the Runaways or the Agents of SHIELD used the time travel technology to revert the snap? Were they that selfish? Due to none of their friends having died to the snap, they don't feel the same need to revert the snap? Or did they, eventually, decided to do it, eventually making an alternate timeline, where the snap was stopped, but the timeline they left behind was the one leading to Avengers: Endgame. What kind of in-universe explanation exist for Runaways and Agents of SHIELD not cooperating in the need of returning the snap victims back to life, when they had the resources to do the same thing the Avengers wanted to do, long before the Avengers themselves?
Wanna hear your thoughts on the subject, but this will be kind of a speculation-esque convo, tbh
