Because it allows the creator to get the benefit when the value of their art goes up and incentivizes them to create more.

I know people here are not big NFT fans but back on my TL, I used to see people publish their mints on fxhash or such marketplaces and actually make a living from it. I'm not sure about the exact price mechanics (there's something called a floor price, # of mints, etc.)

I'm not dismissing zaps as an incentive but blockchains are an alternate means.

I'm sure someone in the digital art trade can articulate this better but as an outsider looking in this is what the NFT piece looked like to me