By going into debt the only thing you are told is "buy it now and pay later" which is altogether too tempting for most humans. It is the same reason why people commit sins and say they will deal with it when they are infront of God. They are heading for disaster and do not have the discipline to say no to temptations.
When living inside your means, you know the budget and plan all of your money. Every dollar is watched and noted into the system. Most people do not think they have the time for it so it is not done. Also most American's do not know when to say no. This is why we have so many alcoholics, workaholics, "big boned" people, etc.