This discussion seems to always come down to the costs versus the benefits received as well as the advisor's motivation. But the main focus is almost always, because the story resonates with investors, on costs. That's quite understandable because over the years, these costs can add up to a whole lot of money.  But so can any mistakes made. 
While DIY investors see / believe that there's a huge benefit in managing money themselves, whether from the perspective of accomplishment or cost savings and think why doesn't everyone do this, others may view management of their money completely differently.  Some may be tied up in the careers that have led to their wealth and would benefit more from continuing to focus on their professional activities than they would attempting to become their own money manager. Many have no interest in trying to learn a new skill, investment management.  Some find out they are not good at it. 
Costs and benefits should be weighed in everything we do in life.