system
Given the following post, respond as if you're commenting on Reddit:
human
[MCU] [Endgame] Why did the Avengers not travel back to go for the head?

Why did the Avengers not time travel to the moment Thanos was vulnerable with Stormbreaker in his chest, and have Thor decapitate him there? If they had, they would have acquired all of the Infinity Stones without the death of Black Widow, and Iron Man. They would also have opportunities for numerous attempts as they are only traveling to one time period. And even better, the alternate timeline they create is safe from the snap, which although doesn’t help them, it does mean this timeline’s people don’t have to live for five years without half the universe.

Edit: The answer that makes the most sense to me so far is that a direct confrontation with Thanos was immediately dismissed as too risky.
gpt
Showing a Thanos with complete Gauntlet that they were travelling from another world? GIGANTIC risk. Screw that up, and you have a fully empowered Mad Titan who now wants to snap *all* the timelines.

Remember, the original plan would have never went directly against Thanos again at all: grab the Gems, snap everyone back, put back the Gems. We'd still lose Black Widow or Hawkeye due to the unexpected snag of the Soul Gem's protection, but the "time travel heist" had the least risk on paper.

It only really went wrong because Nebula accidentally tipped off Past Thanos, which wasn't anticipated at all.