The scammers have the card numbers in a program that alerts them when the card is activated and at what amount, and the programs can immediately redeem them, and transfer the card dollars to a wallet or other source.
YouTube anti-scammers explain this in videos, and do good job breathing the process down into understandable segments for non-tech people, so they can avoid being scammed.
Kitboga
Scammer Payback
Jim Browning - Tech Support Scams
Very good anti-scammer videos