A lot of men die with prostate cancer, because only very few die from it. And if you belong to the former group, knowing about it or doing any kind of intervention means a massive loss in quality of life. So the best course of action overall is to close our eyes and stop looking. And hope you don't belong to the latter group.
Perhaps this plan just needs better marketing. Instead of dividing tumors into benign and malignant we could have a third category for malignant but slow-growing.
(Yes, yes whole body scans exist but these are largely pseudo-medical scams that don't deliver what they promise. I'm saying deliver on it, within reason.)
edit: Ah ok. Risk of over-treatment by broad scanning? "Active surveillance aims to avoid unnecessary treatment of harmless cancers while still providing timely treatment for those who need it." according to NHS.
But if we do scan and test and screen, a lot of men will find out they have it, become anxious about it and will want to do something about it, which leads to a lot of unnecessary treatments that decreases people's quality of life and wouldn't extend their lifespan anyway.
If there was a simple cure for it once detected, we could screen and test everyone all the time.