M doesn’t really do that for anyone but new grad hires. I’m not sure either A does that either (the A closest to M doesn’t, I know from personal experience). F and G definitely do, they are notorious for it.
I'm not entirely sure why, but here on HN especially I've seen a lot of the use of FANG with one A [Apple] used to mean Bay Area super-powers, as opposed to GAFAM for general tech majors, because some believe the Bay Area to matter more than the rest of the industry and Seattle-focused Amazon and Microsoft somehow "outsiders"/"foreigners" to the Bay Area.
It's also fascinating to me from an anthropological/sociological view point because Microsoft has had some sort of Bay Area campus (almost continuously) for longer than several other members of GAFAM have even existed (with PowerPoint headquartered there from its acquihire in the 1980s until a reorg shuffle in the mid-90s and HoTMaiL/Hotmail/Live Mail/Outlook.com and other "web properties" carrying the torch up until a lot of recent acquihires expanded that campus again to more Office-relevant tools beyond just "web stuff").
I didn't invent the FAANG acronym myself, but, having a lot of [ex]colleagues in those companies I can see why people want to work for the interesting and somewhat innovative "FAANGs" and don't put Microsoft (or IBM, Dell, Oracle) in the list.
This happens at FAAMG companies.