I was also initially salty about it, too, but I soon came to see that sandbox has it's reasoning to be locked behind at least a certain point of the story. This is an atmospheric simulator horror game, and sandbox I feel gives far too much freedom to the player to just be unlocked right off the bat.
There is a level of discovery in the game that I feel instantly having sandbox unlocked would potentially discredit. Normally by the time you unlock sandbox, you have likely experienced enough of the game and it's story content that you are "worthy" of having access to the ability to mess around with it. But if sandbox was instantly unlocked, rather than experiencing an discovering everything in the game naturally over time, a new player could just go through it all at once with cheats.
Additionally, sandbox doesn't let you move items outside of sandbox saves without exploits, so you wouldn't have been able to do anything with the spawned keycard anyway. Unrestricted cheat access outside of sandbox does exist as one of the Patreon supporter rewards.