To simplify, the cards must be activated first. If you buy them at a register, the register activates part and loads the money, so you can just steal a stack of blanks and get anything. .... yet, the back of the card has a big sticker with activation instructions. You must go online or call the phone number to activate the card. They probably want your name and address at minimum, as they are pressured by the government for Know Your Customer laws. Some cards even from banks, may allow setting most of that up ahead of time, but they usually still require an initial activation, by using it at an ATM with you pin code (just check balance is all that is required).

Credit cards are super insecure and mostly the way they are to collect your info and spy on your purchases, so they make you jump through hoops. The companies eat literally $100's of millions per hour in fraud, rather than using something more modern and secure.