I'm guessing the same reason owning the rights to song lyrics can be very valuable, despite the fact that the lyrics are easy to copy and paste. You can say in legal terms "I own that" which gives you monetisation options you might not otherwise have in the regular economy.

It's obviously a bit tenuous because of the digital only nature. A real Banksy that physically exists with proof of originality affords control and exclusivity over it's access. A digital Banksy is provably yours as an NFT, but doesn't actually prevent others from viewing it trivially and perfect copies obfuscate the nature of "original". So you own it, and assumedly have exclusive rights to sell it, but it's also very easy to copy and distribute.

I'd be very interested to see if the recent push to artificially inflate the price of NFT's in order to set a precedence for what people should expect as their innate value works.