That's because
Spoiler:
Thanos destroyed the stones in 2018, remember?The stones controlled the uniform flow of time, and were responsible for the creation of alternate timelines. This is essentially how order was maintained throughout the concept of time travelling. However, since there were no infinity stones, the mainstream reality existed separately from the rest of the realities, which is why they needed to use the stones from the past to undo the snap, instead of simply stopping it from happening. Then, when the stones were returned to the correct points in time, it stopped the timelines from branching into alternate realities, and since Cap had just replaced the last stone, the realities merged together since all the stones were back in their new mainstream reality at the correct times.
So basically, the fact that the stones were no longer IN the original mainstream reality is what made the time travel mess as convoluted as it was. They couldn't change the past, because the original reality in which Thanos killed half the universe no longer had a linear past that could be changed, which is why removing the stones simply made new branch realities.
Think of it like this- Thanos destroyed the stones. Meaning they couldn't simply go into the past and kill him as a baby, because that would simply turn that new past into a new branch reality, since it was separate from the reality where Thanos did the snap- because they no longer had the infinity stones. The only way they could have changed what happened, was by bringing everyone BACK with the IG. Once they went and retrieved the stones, they couldn't just stop the snap from ever happening- because that would have created a paradox, since they had to retrieve the stones in the first place to stop the snap from happening, in which case they wouldn't have ever gone to retrieve the stones in the first place, thus not stopping the snap from occurring.
So the only way they could have undone the snap was by undoing the effects of it in their own reality, and then taking the stones back to their places of origin, to restore the balance in the timeline- and merge the realities all together again.
This is what stopped Cap from creating a new branch reality. All the stones were in their rightful places, and the time he was in was one in which all 6 stones existed at once. Meaning he was able to effectively change the past in that small way, since he was already IN the past, and all 6 stones were present at the time.