When the term FAANG was coined, those 5 companies were paying engineers around double what they could ask for anywhere else. I believe that's when software engineers started grouping them together. It became a goal for many young engineers to join because of the fantastic salaries/benefits.
Things have changed since then but the point remains, FAANG generally means "large company that pays above market for software developers."
Things have changed since then but the point remains, FAANG generally means "large company that pays above market for software developers."