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.
https://ja.wikipedia.org/wiki/%E3%83%93%E3%83%83%E3%82%B0%E3...
The guy yelling on TV all the time?
https://www.nasdaq.com/market-activity/stocks/msft/advanced-...
Are you talking about recently or circa 2013 when Cramer popularized his FANG acronym? Before 2014, MSFT growth didn't compare to the newer web dominant companies like Facebook/Amazon/Netflix/Google. He also left out other giant tech companies with lower growth like IBM, Oracle, SAP, Yahoo!, Hewlett-Packard, etc.
U, L, and A are Uber, Lyft, and Airbnb.