It's just because FANG (don't think Apple was originally part of it) was a list of fast-growing stocks, coined by none another than Mad Moneyman Jim Cramer

https://en.wikipedia.org/wiki/Big_Tech#FAANG

Quite honestly that term in both stock and big tech senses is outdated. It should really include a few post-unicorn giants like Uber nowadays.

I’ve seen FAANGMULASS a couple of times.