You can't make as many Bitcoins as you can pokemon/baseball cards/pet rocks. That's one thing backing it. There will only ever be 21 million bitcoins in existence. Ever.
I see your point, but from my perspective it means little. There will only in history be x number of hairs on my head, but that doesn't make them worth anything.
The supply of first edition Charizards (a pokemon card) is also fixed. Any future printings will be a later edition. By your logic, is there a reason we shouldn't use first edition Charizards as currency, since the supply is fixed? (Counterfeiting is the only good argument I see.)
We could fork the software and call it something else. Maybe change the hashing algorithm and the hard limit on the maximum number of coins, like Litecoin did. Trivial to do. There is no intrinsic difference between a Bitcoin or a Litecoin except they are used on different networks and have a different name.
By contrast, precious metals have inherent properties determined by nature. Gold is inert, it has never been recreated by man synthetically, which is what makes it so precious.