When you buy an nft, you but a number

When you buy an NFT, a storage slot in the contract's (contract == dumb Ethereum nomenclature for a program running on the blockchain) is set to your account's address.

This is very similar to how any traditional database works, so nothing new here.

(and since you can't own a number, you buy nothing).

A counterexample: This sentence is encoded as a number. I own the copyright for it. Someone could buy the copyright for it.

Unless you're arguing the semantics of ownership? Like you can't own a number because you don't have exclusive use of it? If that's your point, I'd generally agree. In the case of an NFT, you have exclusive use of that abstract "thing" represented on that particular chain.

The fact that you got scammed because some people told you that this number gives you ownership of anything is meaningless.

If I can exchange my number for something else I want, or it has some intrinsic value to me, it's hard to argue it's meaningless. I have a few NFTs simply because I want them, and couldn't care less about whether anyone else finds meaning in them.

With your example of a dollar, it would be as if you were given monopoly money and you were going around thinking that you're rich. Sure, if you find people that fell in the same scam, they might accept your fake money as payment, but that's the extent of it. It's basically a group of deluded people who imagine that they have something of value while they simply have a meaningless paper.

How do you think regular currency works? Everyone believes in the value of a dollar, so that dollar has value. Imagine you were trapped on a barren island with a stranger. You have a hundred dollar bill, and they have a box of rations. Suddenly your dollars are rather meaningless, no? *

That is also why you can easily print monopoly money, but not real money. One has a value, the other doesn't.

Have you tried printing cryptocurrency? That's kinda its whole raison d'etre.

*: hopefully they share out of the goodness of their heart!