Oh, it's a stock thing? It makes more sense as Wall Street slang - in terms of the technological landscape, one of those companies is so obviously dissimilar to the others that the phrase makes little sense.
The acronym was coined in 2013 to describe the best-performing tech stocks at the time. Microsoft's resurgence hadn't really happened yet. The acronym was so convenient that no one's updated it except to add Apple (which didn't require a change in pronunciation).
I see Big Four (Google, Facebook, Amazon, Microsoft) more often than FANG although that is obviously confusing on Wall St where that phrase usually refers to the large accounting firms. Apple is excluded because it is a hardware company not a software company although sometimes it is subbed in for one of the others, depending on who is using the phrase.