Here is a fairly typical example. My friend has gotten into running and he's been consistent for about two years. He does three runs per week. A ninety minute long run, an hour run, and an interval session that takes about fifty minutes. Moderate volume and a reasonable plan. But consider his times. He usually does about 32 minutes in the 5k and 2:40 in the half-Mary. His PR's a bit better. Why is he so slow? Shouldn't he be running a 5k in under 20 with that mileage?
Now, he is overweight. But say he is 60 pounds overweight. Using the standard "2 seconds per pound" rule, he is about two minutes per mile slower than he could be if lost the extra weight. But that just gets him from doing 12 minute miles in the half-Mary down to ten minute miles. Still slow. He could speed up even more if he went from "not overweight" to "real runner skinny". But that would still only get him down to 9 minute miles, perhaps 8. That would be a 1:45 half-Mary.
Something doesn't add up. Three hours of running per week should make him faster than that. And I'm not singling out my friend because he seems pretty representative for a typical runner.
I'm guessing it mostly comes down to the fact that serious runners have mentally moved the bar of "fast and slow" to a point that is unreasonable for most regular folk.