The main reason that some managers fail to plan is the same reason that some people eat more than is healthy for them or that they fail to exercise. That is, many people like to take the easiest path rather than the path that is best for them in the long term. This appears to be a flaw in our human nature.
Business planning can be difficult. It takes time and it takes mental effort. Therefore, it is not always the most fun thing to do. In addition, there are times when it does not seem necessary. If you do not plan, your business does not fail on the spot any more than you die instantly from not exercising. Therefore, it can seem that there is no point in planning at any given moment. The gains from planning, and the losses from failing to do so, are not apparent until later.
Some managers fail to plan, then, because it is easier not to plan and the consequences of a failure to plan are not experienced right away.
I wonder if American International Group, General Motors, and Kodak and had plans.