Also, think about how hard people find it to lose weight. To sustain the motivation and follow through, day to day, workout to workout, meal to meal. And it's even harder to do that when you have lots of challenges to overcome (lack of time, limited funds, no good grocery store, unsafe neighborhood that you can't workout in, etc etc.).
The same can be said for the process of becoming successful. It's a process that requires recommitting yourself daily...turning in every homework assignment, studying for that nursing exam, ignoring all the people and signs around you telling you that you can't do it, not getting caught up (with having a baby or selling drugs), and not taking the easy way out even though it's what everyone else is doing and has been doing for generations in your family/community.
This life is hard, especially when you're starting in the negative. It's hard to sustain the motivation. It's not necessarily that people don't want it for themselves or aren't willing to do the work. It's just that sometimes, like with weight loss, you can want something, and even know how to get it, but it's just really hard to get there.