I think it's based on a lack of education about bicycling.
I believe this is part of the reason; there's also the larger problem that we as a society tend to see little value in daily physical activity for any reason. Four of my six nephews and godsons participated in sports while they were in high school, but none followed it up with routine exercise of any sort and only two have shown interest in keeping up with outdoor activities of any kind. They're all now in their late teens and 20s, and two of them are already considerably overweight; a third is marginally so. The youngest two still like to go riding with me and I have somewhat more hope for them.