Everyone knows the original artist for Mona Lisa so anyone buying your copy is paying for material rather than artistic value. If you gather arts of obscure-enough origins and sell them off as NFTs, your clientele pays for artistic values you stole, which is a fraud in spirit.

In modern internet, free, high quality, copyrighted but not copyright enforced arts are something that can be sourced at rates of almost items per second, rather than months per item as was in days Da Vinci lived. That makes such frauds extremely easy, and million dollar NFT craze only helps justify it on every aspects except ethical grounds. So it's a problem.