People hosted their own forums as there was no viable alternative, and you didn't care about legal liabilities, data governance, right to be forgotten, DDoS protection etc.
Most people (not all) will choose the easy option of an existing service. Of which the value for that service is to lock you in and spend all your time there.
Saying that I'm still active on multiple forums, but they've been around for years, and it's definitely an older nerdier demographic.
https://www.comma.ai/openpilot looks cool af