Sites like bitaddress.org and Blockchain.info where you can print out a paper wallet also show your current Bitcoin balance on your paper wallet. But your balance is something that is constantly changing, so why would it be printed on a paper wallet?
The balance of an address only changes if you send bitcoins into or from it. Paper wallets are usually intended to be swiped only once, so you should not have outgoing transactions while the paper wallet is still valid. You could add coins to it, as a sort of piggy bank, but there are also use cases where the amount will be fixed.
So, if the paper wallet is intended to be used with a fixed balance, it can be printed on it for convenience; if not and an amount is printed anyway, it can simply be ignored.