Because it's not real. Porn addiction isn't a medical condition. It's just made up by incels who want sympathy from others for making bad choice, and by man-fluencers who are selling toxic masculinity.
Lack of exercise, poor nutrition, bad sleep, and tons of other stuff that 4chaners probably do can definitely ekad to ED though. So it could just be yet another mixup of correlation vs causation.
Used to be they didn't recognize a whole lot of things we now realize are actual things. I would be a bit careful about considering it an absolute arbiter on the matter
While the World Health Organization's ICD-11 (2022) has recognized compulsive sexual behaviour disorder (CSBD) as an "impulsive control disorder",[1] CSBD is not an addiction,[2][3][4][5][6][7] and the American Psychiatric Association's DSM-5 (2013) and the DSM-5-TR (2022) do not classify compulsive pornography consumption as a mental disorder or a behavioral addiction.
It would make sense that a layman would call "impulsive control disorder" an "addiction".