For some time now, every Marvel fan has been primed to understand the MCU as a system governed by rules, just as any expanded universe would be. Only Doctor Strange can use magic, only Ant-Man can grow or shrink to sub-atomic size, only uniting all of the Infinity Stones made the snap possible. Everything that's possible is made possible by some sort of framework because otherwise having a bunch of superhumans and Gods would be chaos.
If all of those characters were suddenly just able to do whatever they wanted, there'd be no real threat to them or to the universe. So in most cases, even the most powerful of characters - or the most specifically gifted like Ant-Man - have limitations on their powers and abilities. Thor's powers are finite, Ant-Man can't shrink past a certain point and runs the risk of losing himself to the Quantum Realm, Hulking out too much makes Bruce Banner lose himself... Even using the Infinity Gauntlet fundamentally changed Thanos.
That's just how you write good drama when dealing with those sorts of characters and Marvel introducing the Infinity Stones was a very clever means of adding control to their story and how you make the possibility of reversing the snap at the end of Infinity War all the more challenging. For the Decimation to happen, a very precise series of things had to happen: Thanos had to be strong enough, he had to make a sacrifice that mattered, he had to unite all of the Stones and he had to be able to snap his fingers.
For the Avengers to undo all of that, they'd have to work against each of those elements, which is why the rumours of time travel seem so pressing. Retracing steps and then preventing them can be the only way to solve that event's tragedy.
But then how can the Avengers time travel without using the one McGuffin we know can reverse time? How can they hope to retrace those steps while Thanos has the Time Stone?