I agree. But given that Vader's reveal at the end of Empire, and the conversation that he and Luke have in Jedi, it would be rather difficult to write the prequels without it spoiling the reveal. You would still have to establish Anakin as a character, in order for the reveal to mean anything. It might have worked had it been written that way, but personally I agree with what Lucas did, which was write it for an audience that already knew what happens later. It wouldn't have made sense to do it any other way.

A few decades down the line if people come to them totally new, it might have worked better had they been written how you suggest, but you can't write things in that way. You have to write for the people who are going to see it on release. And we all knew who Anakin Skywalker was.