Why is a Permanent Resident Card an unacceptable form of ID for age verification?

I don't know why. However, perhaps it is considered a supporting documents, like a birth certificate. You need to go get a state-issued I.D. card or driver's license using your Green Card. ID Requirements for Washing State Liquor Control Board: Identification MUST be valid (not expired) and show: The bearer's date of birth, The bearer's signature (except US Military ID see below) A photograph of Bearer Types of Acceptable ID 1. Driver's License, Instruction Permit, or I.D. Card issued by any U.S. State, U.S. Territory and District of Columbia. 2. Driver's License, Instruction Permit, or I.D. Card issued by any Canadian Province 3. Valid Washington State Temporary Driver's License 4. U.S. Armed Forces I.D. Card (Encrypted signature acceptable) 5. Merchant Marine I.D. Card issued by the U.S. Coast Guard 6. Official Passport 7. Washington State Tribal Enrollment Card (No expiration date required)