I think it's just the first idle thought because they hear it so often. I don't think it's men in general, just a certain group of "old schoolers" who have not learned that all those heavy benchers are injured, unless they have started working other body parts too (hello).
Many men who are not into fitness just know this line. They don't know what a repeater is, though, or a power scissors...
You can smile and say,"Yeah, what do you bench, ha ha."
If a man is seriously asking, you can let him know you rarely do your one-rep max, instead you simply work out regularly. It's a nice answer, designed to steer the conversation.
Edited to add, I guess I can bench about 70 pounds. That is wimpy compared to what many men can bench, I mean much more than their body weight. I weigh a lot more than 70 pounds but I really haven't tried a one-rep max...
By the way no man has ever asked me. Men have said, though, "You look like you're in good shape." That's nice.