The acronym was coined in 2013, when Microsoft arguably looked on track to be the next IBM (the year after, Ballmer departed and their turnaround started). So it kind of made sense at the time, maybe? It’s a bit odd now, granted.
They were, though now I feel Microsoft is actually a more diversified tech company than the FAANGs. Consumer software and hardware, enterprise software, cloud / Internet, gaming, open source (github, etc.)
The acroynm is based purely on stock performance during the time it was created. You could make the same argument for Apple but Apple was on the list from day 1.