If it was centralized, there would be only one blacklist.

A blacklist doesn't prevent someone from posting and earning. It is a warning sign that someone is a bad actor.

There is no police or law on Steem, so it is up to the community to police itself. Most people are unable to and don't want to put the effort in to prevent spam, abuse, harassment, plagiarism, and scams.

Some of us go out of our way and put a lot of time trying to keep the community clean and safe.