The reason isn't unknown; It's always been Option on Mac keyboards since the original. Apple started printing a small Alt on it in the 90s as well for PC compatibility software, but it's always been option. Some keyboards will print the symbol only and have a small "alt" in the upper left corner, but that is an additional label. It should never be referred to as Alt, unless you use it as an actual "alt" key in software that recognizes option as alt (e.g. a VM or a command line).
/pedant
And before some other pedant tries to come in and out-pedant me, option was never closed-Apple; the original Mac keyboards printed the word "option" and closed Apple was on the previous Apple IIe/c keyboards. AFAIK no closed-apples ever showed up on ADB or Macintosh keyboards, even though open-Apple appeared on the command key. There were no standards for open/closed Apple's functions in the Apple II era, so it doesn't map up quite the same until the IIc introduced MouseKeys. Open-Apple often found the same use as the command key today, but that was more a de-facto thing than decreed by the OS like on the macintosh.
Amen, brother.
That said, please allow me to gently but most respectfully attempt to out-pedant you...
You are absolutely correct that the original 1984 Mac keyboard featured only Option and Command keys.
No keys with Apples to be found here:
However, the Open and Closed Apple keys did
not begin life on the IIe or IIc, but rather... the 1981 Apple ///.
Behold, Open and Closed Apple keys immediately next to one another, a caps lock key (this being the first Apple to natively support lower-case characters), and
four glorious
pressure sensitive arrow keys. You read that right, pushing down further on these keys caused them to repeat faster, making your cursor zip across the screen. A real quality-of-life innovation for its time.
For reference, a contemporaneous Apple II+ (1979) keyboard looked like this:
Eventually the IIe arrived in 1983, repositioning the Open and Closed Apple keys to opposite sides of the spacebar.
Curiously, the 1983 Lisa included both Option and a single Apple key, though it was printed as a solid Apple. Note the lack of arrow keys, a decision both shared with and preceding the original Macintosh.
And yes, Open Apple did eventually share a key with Command for many, many years, starting with Apple's ADB keyboards.
Fun fact, this did
not begin on a Macintosh. Instead that honor belongs to:
The Apple IIGS keyboard.
The IIGS being both Apple's first ADB-based and platinum-colored computer. Yes, the legendary industrial design ushered in by the Macintosh II and SE was, actually, first introduced by the last great Apple II.
<FIN>