That's because it's not possible to do so.

Well, almost.

With MasterCard, Discover, and American Express, an authorization is a one-way process. The merchant can request the authorization, but once that money is held, the merchant can't make any modifications to the authorization or do anything other than submit a payment request against that authorization. The only way to void or reverse an authorization is for the merchant to contact the issuing bank directly and request it to be removed. This is a fairly labor-intensive process: the merchant has no central number to call for this, so he or she must get the number off of the back of your card. Then, the requirements are different for every bank: some banks will take the merchant's request over the phone (perhaps after verifying the merchant's bank number, merchant number, and the authorization code and dollar amount), and other banks require a faxed note on company letterhead. Some banks won't reverse it no matter how you ask them, and (most frustratingly) some banks won't even talk to you via the number on the back of the card if you're not the cardholder and can't provide the cardholder's SSN and other verifying information, so it's kinda hard for the merchant to do this without the cardholder calling the bank in front of him or her and getting past that.

Visa's systems are a little more advanced. SOME (not all!) POS systems allow the merchant to perform what is called an authorization reversal directly through the POS device or software. However, even among those POS systems that allow this, I think most cashiers/clerks/CSRs aren't even aware of what it is and may not even fully understand the difference between a payment and an authorization, so I doubt you'd get much help from them if you ask them to void the authorization.