Hi wjfortier
Welcome to the Apple Community.
We are all Apple users, like yourself, in this community,
so we have no way of knowing why Apple does things the way Apple does them.
Guessing you may need to prove you have the money before you can use that money, like at a bank ... ?
:Apple Cash Terms and Conditions:
https://applecash.greendot.com/termsconditions/
How to: Set up Apple Cash – Apple Support
How to: Add money to Apple Cash - Apple Support
says:
"you can add money to your balance using the debit or prepaid cards that you already use with Apple Pay in Wallet.
If you don’t already have one of these cards in Wallet, you need to add one before you can add money".
You can't use credit cards with Apple Cash, and you might not be able to use some prepaid cards.
You can't add money from a gift card, even if it's already in Wallet."
If needed: Apple Cash Support website.
All the best :-)