I love the excuses so far.....hahahahaha
Nobody has gotten the right answer yet as to why people are fat.
There is no specific answer, it's a number of factors, including bad diet, little excercise. Bad diet can be split up in too much fastfood, to much refined sugar, too big portions etc. The reasons behind those factors again have different causes, like lack of time, lack of knowledge, instant gratification motivation etc, sugar addiction. Suggesting that there's one answer is a folly. All this is no excuse for being aboses, but it shows obesity stems from cultural values to self-destructive behaviour to genetics in some cases. And all these factors intertwine and interact...
Besides, being single digit BF is different from being healthy, around 15% BF is perfectly normal and healthy, constantly being below 10% can lead to reduced immune-system functioningand disruption of the hormonal system, among others.