It's partly because of the kids.
It's also because social media is part of the USA's soft power projection, and many of us now consider this to be a threat.
It's also because social media has a long history of manipulation for their own gain, against the users' interests, dark patterns, tracking, they fail to back down from and even file lawsuits to continue tracking when tracking itself required (under GDPR etc.) permission that e.g. Meta did not have: https://9to5mac.com/2021/01/28/report-facebook-building-anti...
For "about the kids", consider: given kids have no direct purchasing power, what adverts can they possibly respond to in a way that actually provides gain for the buyer of the advertising slot? They cannot. Therefore, by fighting for the right to keep kids on their sites (despite the huge extra effort that needs to exist to keep them safe on their sites given the inherent ambient hostility that comes with giving everyone direct access to, in Facebook's case, a few billion other humans), at least one of two things must be true: (a) they think they can get kids hooked, and be able to convert them to profitability as adults, and/or (b) they are scamming the people who buy advertising slots, knowing full well the kids who see the ads cannot possibly buy anything. If a third option exists, I cannot guess it.