> It's because they make their money from the hundreds of billions of dollars they skim off the top of their customer funds.

The movie Trading Places nailed this, albeit in the context of commodities rather than stocks. But the principle is the same:

   Mortimer Duke: Tell him the good part.

   Randolph Duke: The good part, William, is that,
   no matter whether our clients make money or
   lose money, Duke & Duke get the commissions.

   Mortimer Duke: Well? What do you think,
   Valentine?

   Billy Ray: Sounds to me like you guys a
   couple of bookies.

   Randolph Duke: [chuckling, patting Billy Ray
   on the back] I told you he'd understand.