Even people that you may think would manage their own investment portfolios don't. In his post Portfolio Management & Decision Fatigue, Ben Carlson noted that even the great economist and Yale professor Gary Shilling does little in the way of managing his investment portfolio. Carlson touches on two very important point that I remind people often when discussing whether or not they should work with an investment advisor.
An individual on their own can have just as much success as a professional advisor with the right blend of discipline, focus, and responsibility. It is just a matter of asking if they want to bare those additional burdens?