A quote I hear often in the management world is "A players hire other A players; B players hire C players". The message is clear - great manager hire great people.
I believe you should go one further: great managers hire people smarter than you. No offense, but it shouldn't be that hard.
The scope of a manager's responsibility is many times quite diverse. It would be unreasonable to think that a manager would be the expert in all the areas. Imagine an HR manager - the people on the team are dealing with recruiting, employee relations, compensation, benefits, etc.
A great manager hires people who are experts in those areas (or who are good in those areas and are being developed to be experts).
Also keep in mind that skills and knowledge is becoming obsolete faster than ever. Knowledge and skills a manager had 5 years ago isn't as helpful. Early in my career I was an instructional designer - I did it with a great deal of success. If I had to do some instructional design work today, I could, but the profession has changed a great deal. I would be seen as a relic. The software, the tools, and the concepts have grown and I haven't.
Always strive to hire better than you, give them room to succeed, and learn from them.