Great question — answering as the UphillTech team:
I get that she's "superheroing" somewhere else, but wasn't this an important enough mission worth calling her back to Earth? She simply wasn't there (or apparently available). It's already established that she has responsibilities to other planets in the galaxy and wasn't available for the first "unsnap". Having obtained all the Infinity Stones there's no point in waiting to get the job done (even assuming, as has been mentioned elsewhere, Carol could have withstood the backlash of the unsnap) and so it fell to Smart-Hulk to do the job. Of course, then Thanos crosses through time and Captain Marvel returns, just in time , to make a pivotal change the the final battle.
Full answer by Paulie_D: https://movies.stackexchange.com/a/102576 (CC BY-SA 4.0).
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