That is indeed a big problem with NFTs.  There are lots of other problems as well.  What it boils down to is that an NFT is just a url on the blockchain (sure some of them go deeper than that, but lets keep things simple).  That url can be changed to point to nothing, the image at the end of that url could be removed or replaced with anything...  IPFS solves one of those problems, and I'm sure contracts could be written to solve for the first problem as well, but most people aren't even aware that what they're buying isn't an image on a blockchain, but a token with a url attached.  It's up to the community to enforce abuse, and sometimes marketplaces step in, but in the end things are worth what people will pay for them, and some people wear fake diamonds, so you can expect that some people will display fake NFTs (not being from the original creator) as well.