Apple famously doesn’t care about things that are non-default settings/customizations/off-the-beaten-track. The standard Apple keyboard is a 60% keyboard (Bluetooth or laptop). And on those keyboards, fn is in the bottom left corner, which is fine for keyboard shortcuts.
Direct quote of parent giving the reason: "if you're using an external keyboard that doesn't have a Fn key built by Apple, there's no way to go fullscreen from the keyboard"
