> Why can't the host of the image just delete the image?

If it's a blockchain, there's no single host who can do that.

The receipt is on the blockchain. The thing the receipt applies to (generally) isn't. The thing can be destroyed without the receipt being altered at all.

Only if the nft project host the image on ipfs. I wonder how many nft projects do that?

I level mentioned anything about images. If you have a LoL skin and the game shuts down it similarly becomes useless. Digital items becoming useless is nothing new. At least with NFTs they can at least be collectors items that you can keep if you want.