The Apple TV app is supposed to write some preferences to disk when the app is closed, including the current signed-in user, so it can load that again at a later time. Users shouldn’t have to sign in each time, unless they sign out, reset, or delete and reinstall the app.
- Verify that your TV system software is up-to-date.
- Verify that the Apple TV app on your smart TV is up-to-date, or set to update automatically.
- Try a ‘cold’ restart of the TV system. (Really off for a moment, not standby-wake.)
Please contact Apple Support when your experience is different, as then your copy of the app isn’t working as it should.