Engineers are incentivized to increase profits for the company because impact is how they get promoted. They will often pursue this to the detriment of other people (see: prioritizing anger in algorithmic feeds).
Doing Bad Things with AI is an unbounded liability problem for a company, and it's not the sort of problem that Karen from HR can reason about. It is in the best interest of the company to have people who can 1) reason about the effects of AI and 2) are empowered to make changes that limit the company's liability.
It takes quite some knowledge and insight to tell whether someone in the AI team, or, better yet, the entire AI team, is up to no good.
It only makes sense for the bosses to delegate overseeing research as sensitive as that to someone with a clue. Too much sense for Facebook.