Unsure why Zwift is so far behind with the log in process if and when you are using Apple TV on a television. When using an apple TV attached to a TV, the app updates regularly which requires you to log in again. In any other situation with any other app that I use (Netflix, Max, Hulu, etc) they provide a QR code or opportunity to log in using your phone. Zwift on the other hand requires you to type it all in. To make matters worse, you have to use your Apple TV remote which is a joke. It is very difficult to type that in every time with the apple tv remote. Why aren’t the QR Code or phone log in options available? You would think this is an easy fix…especially with the amount of money coming in from monthly subscribers. Anyone else see this as a problem that could easily be addressed?
I have no idea how easy this would be, but it’s a good idea and would significantly improve the experience for Apple TV users. 
Excellent idea!
I use Apple TV and when the app updates there is simply a blue dot next to the app indiciating it updated. I have not been forced to re-enter credentials in a very long time. Perhaps i misread your issue.
About once a year or two, Zwift on AppleTV goes wonky.
Do a full RESET & UPDATE of the ATV - this will wipe it, and update tvOS. You’ll need to reinstall your apps, including Zwift, but this should be pretty easy if you’re using an iCloud account.
On an ongoing basis, ensure you force-close the Zwift app after every ride. (not every session of multiple rides, but every saved ride, prior to re-riding)
Doing the above, I haven’t had to re-enter my credentials in months, maybe over a year now. (I ride nearly every day, almost all on ATV)
If that doesn’t solve your issue, the only other thing I can think of is there’s some type of permission you need to grant in order for the app to remember your credentials. ATV is pretty secure so anytime it asks if you want to allow something (nearby devices etc), I’d default to “yes”.
Authentication can be a tricky enough task but i agree it would be incredibly worthwhile to have this functionality. Especially in my house with 3 zwifters.