>> The rank and file engineers and designers and PMs doing the work were all morally correct people
> Then why didn't they quit?
I can't speak for those people, but decisions like quitting your job aren't one-dimensional. Quitting a job can create severe hardships for a family, stress relationships to the breaking point, etc. The Bay Area is very expensive, so I'd imagine that's true even for workers making otherwise very good salaries at Facebook. Then consider that the harms caused by Facebook are remote, abstract, and diffuse. Also, making the pushing for the right decision to only get overruled by the people in power can feel like doing enough for many people.
Also, there's the frog in a pot effect: a lot of the problems with social media have only become well understood relatively recently. Maybe you'd chose no to join Facebook today, but would stay if you'd already worked there for 10 years.
> If I can choose to avoid working on morally compromising things, why can't they?
What other morally compromising things have you not chosen to avoid? The modern world is full of them, many of them well publicized even if they're not in-your-face. IMHO, it's impossible to avoid them all without killing yourself.