They aren't paid to do. They are paid to be. Somebody with hiring authority thinks that business analysts are needed, so business analysts get hired.

As the article says, not all of them are useless. Some business analysts provide very valuable insight, and a manager who has come into contact with one of those will want to hire more of them. By the intangible nature of their value though, it can be hard to judge whether you've hired a good one or a leech.