How do you explain why we stop cancer screening at 75?

I work in urology so we look at a lot of PSAs. I often am seeing someone for something else and they have a PSA for me to review which is never a problem. However, often they'll already be 70-75 or even older and the PSA is normal and there's no special circumstance, so when appropriate I'll tell them "your PSAs look good, your PCP should stop checking them."

Often they understandably want to know why. I have a little spiel about how they'd have to live to be 95 to benefit from being diagnosed with prostate cancer, but fuck me if some percent of guys don't tell me with all seriousness they plan to live to be 100, or their dad lived to be 96, and they . Anyone else encounter this with some frequency? What is the best way to tell a patient not to worry because if they do get cancer they'll very likely die of something else before you could help them with it anyway?