It's not lost, it was an allegory. Plato didn't hold knowledge of a lost civilisation that no one else did, but he did create stories to make people think and to prove his point. It would be like if I told you that there was a great land of Reddites where the civilians all paid for products with not money but their words, and the best words gained the best money, so people began to copy each others words word-for-word to try and make the same amount of money they all got, but this just created a world where no one knew how to make an original argument anymore and it descended into madness and fell into itself. In fact, I stole that example from someone else.

But it's a good description. People in antiquity were made fun of for thinking it was a real place. It was only in the last couple hundred or so years that people really started to miss the context of the story and begin to think it was real. Just like if my post about the land of Reddites was found 1000 years later away from all the other context I provided people might think that I was describing a real place. But most people who argue that Atlantis is real can't read Ancient Greek, aren't familiar with Plato, and haven't actually studied the topic well. There's a reason for that.