It's not just bloggers. There are computer scientists with publications in top CS conferences that post on LW. For example, Alexander Turner, Dan Hendrycks, Paul Christiano, and Jacob Steinhardt all post there.
Then the question you should be asking is why choose to post on LW when the value of publishing a paper is substantially higher.
Perhaps because what they post doesn’t hold up to scrutiny from their peers, so they prefer to play in the mud with the folks who won’t notice the issues with their ideas.
Perhaps because what they post doesn’t hold up to scrutiny from their peers, so they prefer to play in the mud with the folks who won’t notice the issues with their ideas.