Because the original context in 2013 of Jim Cramer popularizing "FANG" / "FAANG" was growth and even though Microsoft was already one of the biggest tech companies, their stock price wasn't really growing like the newer FANG.

https://hackapedia.com/terms/f/faang

Others later tried to revise the acronym to include Microsoft with variations like FANGAM -- but it doesn't seem to gain any popularity in internet discussions or mainstream media.