Why can’t gift card be replaced ????????????????

Hi Im not happy with the service I got I traded in my phone n was given a gift card....I bought a few stuff n I left abt 400 on the card... unfortunately I lost my wallet with the gift card n went to one of ur store just to be told it can’t be replaced ????????this is unfair though... I thought the gift card was linked to my iD moes..why can’t it be replaced...