The same reason we don't screen for pancreatic cancer or esophageal cancer or brain cancer. The yield isn't worth the testing for rare forms of life-threatening cancer. Also, its estimated that 80% of men over 80 have evidence of prostate cancer. So, if you test all 80 year-olds and find prostate cancer in 80% of them, you can imagine what undo chaos that might cause. People that age will fret, because people don't understand that doing nothing is the best thing, rather than some unnecessary operation that would not have an impact on their longevity, but might kill them or worse (like wearing a diaper for the rest of their life). Its a hard concept to understand. "I have cancer and we aren't going to treat it?" Even some doctors don't understand, like someone here said their doctor continues to screen them because they are "thorough".