Because as a security mesure, they are deleted once you remove the passcode.
Page content loaded
Finally figured it out on my own!
For anyone also wondering, you can turn the passcode feature off and back on but don't actually lose the cards previously added. They aren't "deleted." What happens is that the cards are kept on file. So when you come back to the Wallet app, you won't see them, but click on the "+" icon to "add a new card" and the cards on file will be displayed - you will be required to enter the security card for any card you want to activate again, without having to take the photo of the card, add the expiration date it doesn't recognize, your name as it appears on the card, etc.
Once you enter the security code, it again contacts the card issuer, then validates the card, and you're good to go.
My confusion came from having them no longer displayed straight off, after turning the passcode back on. I hadn't realized that "adding" a card again doesn't require the complete process -- only verifying the security code to retrieve information previously entered from the card on file.
I hope I've described this well enough for others who might wonder in the future.
Thanks for the heads up, dmhmt. This must have changed in iOS11 as before, if you put the phone in lost mode or turned off the passcode, the cards would be deleted, and you would have to add and reauthorize them all over again.
Good to know this fact. Thanks again for adding and updating this important tip/info.
Question: How do I keep the added cards in Wallet when turning passcode off? Is it designed to make you Re-add over and over?