Dr. Katz has an extremely high patient turnover rate.Most guest stars on the show only appear once; a few appear two or three times. Only a select few (Dom Irrera, Ray Romano) appear frequently. This could imply that Dr. Katz loses a lot of patients, which could be because he's an ineffective therapist. That said, he somehow manages to keep drumming up business, so he must be doing something right to keep his office running. And it should be noted that pretty much the only reason Dom Irrera keeps returning is that he enjoys Dr. Katz's company; he still thinks Katz is a lousy therapist and calls him out on it.
Many of the guests were forced into therapy by a loved one.Many of Dr. Katz's patients are stand-up comics who, instead of seriously talking about their personal problems, prefer to do their act. While it may seem odd at first, think about it this way: Someone close to them, be it a spouse or a family member, urged them to go into therapy. The patient felt they didn't need therapy but begrudgingly attended. But out of protest for having to attend, they didn't open up with Dr. Katz and merely did their act to pass the time.