Why can’t I store my cards ccv number in wallet

If you keyed in the CCV number even if it's the same and tap done,

you will get an notification e-mail from Apple informing you that your Apple ID information has been updated.

NOTE: Apple always addresses you with your Full Name they have on file

not Dear Customer, Dear Client, Dear your_e-mail_address, Dear Apple User, ...

this is how you can tell apart from spam/phishing from criminals.

Well it's a security measure.

You also cannot see the card number (except the last 4 digits), expiry date, ...

You don't need it when using, unless you want to edit/look at it.

If your credit card is valid and you've set it up in Wallet,

all credit card info including CCV number has to be entered and is stored within Wallet.