Part of the reason that managers (or owners) fail to plan adequately for a business is because running a business is complex and requires lots of different types of planning in order to succeed.
Consider a manager who must staff all shifts adequately. While she believes she has planned for this, three employees contact her with a flu diagnosis and inform her that they will miss several days of work. The manager begins to shift planning, dedicating more time to filling these now empty time slots and fails to plan for the supplies which need to be ordered for next week's production. Unexpected events that interrupt even the best plans can impact business plans in other areas.
Planning also requires great insight into what the company needs. Managers are expected to evaluate the needs of the company, the mission it espouses, and the projected outcomes based on current company functioning. If a manager doesn't have the experience or foresight to evaluate her specific company's needs, it can lead to poor planning.
Sometimes managers fail to plan adequately because they don't value the insights of their subordinates. Those working on the "front lines," so to speak, often see needs that managerial staff cannot. If managers do not value these contributions and instead adopt an egotistical mindset, they may miss crucial information that could help them better plan better in specific situations.
Good managers are trained in the specific needs of their companies, are given time to evaluate various areas of planning needs, and stay in touch with their subordinates to determine how planning needs to be modified over time.