Kids goto school 9-3 (or something close) to those times). They have PE class twice per week as part of the 9-3. We can argue that kids a long time ago used to play outdoors a lot and these days they play computer games, but the point is, 6 hours in a day, including a break time, a lunch time and PE time.
And then we grow up and goto work.
The hours are longer, the breaks are shorter, the lunch is eaten at the desk. Problems start.
Maybe you have to do some after-hours work, maybe on the day you were going to the gym. Or you're too exhausted after work to even think about walking to the shops, "I'll do that next time, but for now I'll take the car."
Now I'm not saying all diabetes starts like that; as some of you have mentioned, it can start as a side effect to other medications and so on. But I think a lot has to do with Western Lifestyle.
Edit: fixed typo