Because you don’t want a food poisoning party for twelve. The refrigerator keeps your turkey at a safe temperature—below 40°F. This means bacteria have no reason to show up and multiply. They stay sleepy. You stay un-sick. Win-win.
Also, the texture stays amazing. A slow thaw means the meat stays juicy. A rushed thaw? That’s how you get dry, sad turkey. Nobody wants to apologize for dry turkey. Nobody.