Why do business leaders accept a success rate of 30% for transformation or change projects?

Over the past 20 years the rate of success hasn't improved (as shown in work by Covey, McKinsey, KotterI) in spite of research, management education etc etc. Is it because it's just really hard, or does change lack the structure and processes that other activities have adopted - or can't change be managed and lead in a more effective way?

Regards

Craig