Gamora and Black Widow are both sacrificed for the Soul Stone. Thanos throws Gamora off a cliff in 2018 and Black Widow, after a fight with Hawkeye, dies the same way in 2023. Both die before the Snaps happen in their respective timelines. In Endgame, the Gamora of 2014 ends up up in 2023 when Thanos uses time travel to get there. Why can’t The Avengers go get a Black Widow the same way? It didn’t seem to affect anything because The Avengers were successfully able to bring everyone back who was Snapped away in 2018. There’s no reason why a Black Widow from 2016, for example, can’t come live in the 2023 Endgame timeline.