It's because it doesn't just have a version of it. It has the original, authentic painting drawn by Leonardo da Vinci. Whether new painters draw the exact same painting, using the same palette Leonardo used in 1503 or not, they can't recreate Mona Lisa, but only replicas.
You took the example too literally. Imagine if you could pull the painting Leo drew in an alternate universe to our universe 
To be more precise without an example it means everyone can get the exact same copy of the the digital art that they sell as NFTs.
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You can't really deny that there's not even one valid argument in favor of NFTs.
Ok. I concede. But in my view when you are giving up something you have to gain something significant in return. If we are going to endure the hardship of decentralized blockchain then the reward we are getting has to be worth it. I do believe it is worth it in case of bitcoin since the utility bitcoin provides is so valuable and useful that it justifies all that but I honestly cannot see the same benefit provided by projects known as Smart Contract Platforms that nowadays focus on NFTs.
I hope some day things change and I start seeing real utilities though so that I can change my mind.