1) the people in this sub-industry role attend conferences and network and make friends and build professional relationships (for job referrals if there are layoffs at one tech co but hiring in the same area at different tech co)
2) at conferences, in group chats (w/ friends made at other companies in previous roles or friends from said conferences), common news events, developments in the field, and trends are discussed and ideas bounced off each other, which means even if there's no top-down order, there's sort of an averaging-effect and consensus building and battle-testing of ideas and approaches
3) there's a revolving door of people who work at one company in this area developing real-utility experience that translates well to working at another, so people move about and policy ideas cross-pollinate and spread. if you plucked someone off the street, even if it helped ideologically diversify the pool, they would truly be less effective as they ramped up and learned.
You don't actually need anything nefarious to happen, just an insular, small community with similar educational backgrounds, shared values, etc. Not a lot of Repubs from Alabama decide to attend Barnard and study _relevant_humanities_field_slash_law_ and then join tech to try to fight injustice/CSAM/bias/misinfo/election interference at the right time during ZIRP and then build the resume to then continue to land those roles.
These people are all doing what they think is right, they just aren't a super diverse group ideologically and there are social channels that tend to create more cross-brand homogeneity.
And even if you disagree with their moderation policy, there are huge, non-controversial areas you never see or think of that they are fighting everyday too (anti-terrorism, anti-suicide, connecting victims to resources, anti-CSAM, etc).
Probably the same way in physics departments, String Theorists hire other String Theorists and over time it grows and grows and grows. Or how economists all arrive at modern monetary theory.