"time in the gym" is way easier to measure than tangible improvement. It's not surprising that people try to optimize things that are the easiest to graph in Excel :P
You're right I think, but what's actually interesting (I think at least) is that time in the gym is more likely to make you a significant social member of a certain group (gym => office, lab, bar, etc).
Time spent present is easy for the social group to measure. Everyone might be impressed by the guy running the 4 minute mile, but after he leaves we're much more interested in Greg improving from his 7 minute mile to a 6:30 minute mile and that 4 minute mile guy was kind of a jerk anyways. Finding a way to get a 4 minute mile without the trackmaster 2000 and the protein shakes, who does he think he is.