Please explain why does prostatitis last so long?

Because... Prostatitis has been one of most misused daignosis, and its context has evolved and changed over time especially over the past two decades, resulting in coining a new term, chronic pelvic painful syndrome/disorder. The DX of bacterial prostatitis is made on positive bacterial culture from post-prostate massage prostate fluid and urine. So, most cases are mistreated. For detail, see doc timely.