Well, we already know the answer as to why the Roger pirates didn't do anything. They were too early, and Roger was dying.
Roger did do something however, as he knew someone was going to come along and finish what he started, so in his dying breath he sparked the Great Pirate Age. Now we know the reason why he did that, as it was to draw out the person who would one day put an end to this Ancient War.
Why didn't they warn the World though? Could be a few reasons, but I think the simplest one is that they were Pirates, and no one would have believed them regardless. Even a renowned scientist like Vegapunk broadcasting it to the World has people doubting him. Who would believe the most infamous pirate crew to ever live?
Roger did do something however, as he knew someone was going to come along and finish what he started, so in his dying breath he sparked the Great Pirate Age. Now we know the reason why he did that, as it was to draw out the person who would one day put an end to this Ancient War.
Why didn't they warn the World though? Could be a few reasons, but I think the simplest one is that they were Pirates, and no one would have believed them regardless. Even a renowned scientist like Vegapunk broadcasting it to the World has people doubting him. Who would believe the most infamous pirate crew to ever live?