I don’t know why so many on the left still post and have accounts there.

For some it's a public relations platform and can't simply have one such as having only a Bluesky account. Others rely on it as a pure newsfeed as much as RSS. Where I live, Facebook is an evil necessity as it has taken many of the functions and services that were previously available for Internet usage, into a single platform.

Also, not paying for that blue mark.