Well, I could manage $10-$20MM (or more) on my own, too, but when the nest egg gets to be that large, it becomes a simple matter of freeing up more of your time. When your passive income gets to be so high (say, $35,000/month from a $12 million portfolio) that you have, essentially, unlimited discretionary money to spend on whatever you want, wouldn't it make sense to pay someone to manage your money and free up some of the time and "brain space" being devoted to personal financial management? IOW, you use a little bit of something you have an excessive amount of (money) to free up a little bit of something you have a very limited amount of (time).
Using the exact same reasoning, if I had a $12 million net worth, I would hire a maid to come in and clean my house, wash my clothes, do the dishes, etc. several times a week instead of spending time doing those kinds of drudgeries myself.