“If it were easy, someone would have done it.” Maybe not.
Maybe the thing is indeed easy, and has been done before. Then someone was the first to do it. The warning that it had been done before didn’t apply to this person, even though it would apply to the subsequent people with the same idea.
This reminds me of the story of two economists walking down the street. They notice a $20 bill on the sidewalk and the first asks “Aren’t you going to pick it up?” The second replies “No, it’s not really there. If it were, someone would have picked it up by now.”
Sometimes a solution is easy, but nobody has had the audacity to try it. Or maybe circumstances have changed so that something is easy now that hasn’t been before.
Sometimes a solution is easy for you, if not for many others. See how much less credible the opening sentence sounds with for you inserted: “If it were easy for you, someone would have done it.”