Spoilers for Endgame [as well as a few other MCU movies] below!
In Avengers: Endgame, we see the Avengers and their allies
Use Time Travel via the Quantum Realm to go back to the past and collect the Infinity Stones to undo the snap in the present.
In the movie, it is explained that
The rules of Time Travel in the MCU work in such a way that traveling to the past and altering something while in the past does not change the future, but rather, doing anything in the past creates an alternate timeline in which said events happen, leaving the 'main timeline' unaffected.
By that logic, would it be possible that the Avengers [or anyone else with the tech required] could
Time travel to the 'past' and bring a character from that past into the future? For example, could Thor time travel to right before Thanos attacked the Statesman and bring the still-alive Loki back to the present? Could they bring Tony Stark back? Maybe Black Widow? Vision, Heimdall, Yondu, maybe even characters like Ronan, Ultron or Thanos if the person with the access to the tech had more malicious intent.
One counter to this question could be that
The Avengers would not do this as bringing, say, Vision back in their timeline would remove him from another timeline, meaning everything happens the same- one timeline has Vision, another doesn't. Additionally, if they brought, for example, Tony from another timeline into their timeline, he would no longer be there to kill Thanos or do anything he does or, depending on when they take him from the other timeline, not be there to do any of the things he does in the time he spends as Iron Man in these movies.
But that doesn't excuse the fact that it's still possible, and I'm sure someone less heroic than the Avengers would have no trouble
Stealing something/someone from one timeline to get it in their own one [Case in point: Spider-Verse's Kingpin]
So is there a reasonable explanation given how the MCU works as to why the Avengers wouldn't do this? There's definitely a chance there was a reason this wouldn't work outright stated in the movie and I completely forgot it, but I don't remember one.