First, my frivolous answer: Because with 28 years of experience at being a CEO what the heck else can I do?
Serious Answer Part A: Very few people care about helping a CEO. It really is lonely at the top. Most everyone wants something from the CEO. Employees want their job or a promotion. Advisers want to get paid and they want more business. Even the Directors want to keep their seats and their real responsibility is to the company, not necessarily the current CEO. Oftentimes I really help a CEO and they really appreciate it. As all humans do, I like being appreciated.
Serious Answer Part B: The way I see it, when I help a CEO, everyone benefits. A CEO who makes a lot of good decisions grows the organization, provides more jobs, has more satisfied customers, and generates more tax revenue. A CEO who makes too many bad decisions ends up with fewer employees, fewer customers and contributes less to the overall economy.
Serious Answer Part C: Because I am a "business junky". I love this stuff. It is energizing to me to make a difference in that CEO's decision. Because through that decision a lot of people may be affected.
There are probably a more Parts to a complete answer because...life is complex. :-)