The average employment length at a faang is 2 years or less. A job in a single top law firm is an endurance battle that can span your career. Switching firms is looked at differently a lot of times negatively.
Once you land that job in finance or as a lawyer you are on track to be set for life. At a faang after a few years you'll get pushed into management or pushed out.
You are just as "set for life" in big tech as big law. In fact, if you're looking at the last decade, big tech won hard considering tech stocks and how big law froze (and even slightly cut) salaries during the great recession.
Once you land that job in finance or as a lawyer you are on track to be set for life. At a faang after a few years you'll get pushed into management or pushed out.
It's not the same.