Why does Apple issue Gift Cards on Trade Ins instead of crediting card used to purchase?

You can chose either:

Apple Trade In - Apple

"Trade in your eligible device for credit toward your next purchase, or get an Apple Gift Card you can use anytime."

"Trade-in value may be applied toward qualifying new device purchase, or added to an Apple Gift Card."

I have never done a trade in or bought a new phone from Apple so I don't know the details. Perhaps you have to do things in a certain order or make sure you specify you want to get credit towards a new device.

You can contact Apple, but remember it has been 3 years since this took place.

Contact Apple. Click here --> https://support.apple.com/choose-country-region Select your country, then a product. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.

or:

Contact Apple for support and service --> "See a list of Apple phone numbers around the world."

Click here --> https://support.apple.com/HT201232