Some 80-90% of men have some problem with their prostate gland as they get older. Beyond about age 60 it is unlikely that men will need the prostate to contribute to seminal fluid, so there seems little point in keeping this troublesome organ - yet a huge proportion of older men end up with either benign prostate hyperplasia and/or prostate cancer. So why isn't the standard treatment just to remove the prostate altogether?
Your question is along the lines of "Many women will develop breast cancer, especially later in life. Why don't they just remove their breasts after menopause, because they won't need them for breast feeding?" There are a lot of disadvantages and only questionable advantages.
Chief among the effects of removing the prostate are urinary incontinence and erectile dysfunction. That area of the body is very highly innervated, and it is nearly impossible to just remove the prostate without damaging anything else. While it is true that many men will develop prostate problems as they get older, they can frequently be treated medically, or just observed for any dangerous changes. Benign prostate hyperplasia is just that - benign - so there's no need to undergo a major operation with significant side effects. Additionally, many prostate cancers are so slow-growing that they never metastasize or pose any threat, so they are simply monitored and treated medically as needed.