In my experience (not of losing a child) it was bitterly lonely and isolating being bereaved. Others less close were saddened by it of course but it didn't dominate their every waking or sleeping thought, like it did mine. (Understandably, not their loss).
If I had been on social media and felt I could ease my loneliness by creating a space to be heard about my loss, validation and sympathy, I can imagine I might have spent a lot of time reaching out on SM.
CY and CG have created a much much bigger stage to do this on than most people do, so it's much more exposing when they do this, from the external perspective. But it's not hard to understand why bereaved people might want to.
Not saying it's a long term helpful action for anyone but just think it's not uncommon especially in the worst most raw stages. SM may facilitate the need to talk and talk about the loss. And it's there 24/7, which often other people to talk to are not.
At that point you may not be in a good state to be considering what the long term down-sides of creating an online conversation may be, vs having a private chat with someone understanding IRL- which may not be available to you anyway.