I work in a shop that servicesthe hardest working bikes imaginable. They rarely break frames and are but are heavy entry or mid level bikes with atrocious maintenance schedules.. on the flip side I rode a 1983 Trek for 8 years until recently. Broke the frame 3 times and fixed it twice. Why did it break? Well I was asking a lot of it: loaded touring, blue MTB trails, commuting virtually every day, groceries, whatever. So I wasn't so bummed when the head tube developed a crack, would rather have been riding a light bike that whole time and then break it than a heavy bike that kept ticking. But most people wouldn't enjoy or understand that, and they buy appropriately heavy bikes, and the frames don't break.