Why I can’t add my air ticket to Wallet ??

From the article linked above:

”If you don't see an option to add your boarding pass, ticket, or other pass to the Wallet app, contact the merchant or company that issued it to make sure they support passes in the Wallet app.”

I know that with American Airlines and United, I cannot add my ticket to Wallet, I can only add my Boarding Pass when that becomes available, which is 24 hours before the flight when you check-in online.