Because changing the past doesn’t change the future. The 2014 Nebula was from an alternate universe, a universe which the Avengers created when travelling back in time.

When you travel into the past, you split the the timeline into two histories. One which is the timeline that you lived in, and one that is the timeline you altered. When you travel back to the present, you continue living in the original history.

If you watch Loki and What If…? on Disney+, the alternate timelines become a lot easier to understand.

Hang on, if Cap went back to put the stones where they got them, it would be tricky. He would have to shove the aether back into Jane, he’d have to give the mind stone and Sceptre back to hydra and put the orb with the power stone into Quill’s bag or “The Temple of the Power Stone” as Rhodey calls it. If Cap throws back the soul Stone into the pond, he might just get Nat back because as the Red Skull floating guy says, “a soul for a soul”. Or he just kept the Stone for himself which is HORRIBLE.

I don't think bringing the soul stone back would revive her, if that were the case the sacrifice would be pointless.

What if? Has come out?!

Dude, I am sooo gonna check it out

An hour ago

@ReigningChampsRazer About your point about Nat: They still used the Soul Stone twice. That still comes with a price. You could say they rented the Stone.

It came out an hour ago?

@TomN2 Loki brings more questions than answers, under my judgement. I would advise to just think about Avengers: Endgame in isolation when deciphering its own timeline issues, because Endgame predates Loki, and Endgame's time travel rules were not considering what Loki eventually estabilished for the rules of time travel in the MCU. Loki just messes everything up.