Because it's not (strictly speaking) accurate. Even ignoring the whole "people say"/"weasel words" part of the OP, the whole "it takes minuscule effort to stay in shape, ergo people who aren't in shape have no excuse" is simply off-base.
First, remember than they're talking about an hour of strenuous activity. Next time you go to a gym, count how many minutes of actual strenuous activity any given person does in an hour at the gym. I'd bet it's less than half. Between changing machines, getting water, stretching, warm-up, cool-down, you're getting not much at all of a real work-out. So, it's not just "an hour a day" it's more like "two-to-three hours at the gym, which means an hour of strenuous workout".
Second, remember that half of all statistics are made up. If you take a random sampling of any population, you're going to find the most common answer. But, that only means that that's mean answer (arithmetic average). If we assume a standard distribution (which we can't), it doesn't even mean that the lion's share of people are anything close to that average. It may only take one hour for some folks, for others it might take three. If you're one of the three-hours-per-day people, that'd work out to nine hours at the gym, which is basically like a full-time job. We accept that some people get the benefit of having a "better" metabolism (technically, it's a counter-intuitive thing, since the benefit comes from having a more inefficient metabolism), which means we must accept the existence of people who have a more efficient (and hence "worse") metabolism.
We can argue about the personal responsibility thing if we want, but I find that discussion boring now. Instead, let's simply accept that the pseudo-science behind any categorical imperative-type solution to weight problems is crap.
Incidentally, I do two hours of karate three times a week, and one hour once a week. That should be seven hours, which means I should be much more in-shape than I am. Either my karate dojo sucks (which I doubt), the actual math is misleading (the whole "an hour of working out isn't the same as an hour of exercise"), or I'm somehow immune from the effects of exercise.