Most of the time, it's because the app hasn't added support for ENS (Ethereum Name Service) yet. ENS lets apps turn a readable name like yourname.locker into a wallet address or profile. If an app doesn't use ENS, it can't read ENS names at all, including .eth names, the most widely used Web3 extension.

Web3 is still young, and name support varies from app to app. Many leading wallets and dApps already support ENS, and more add it over time.