A big part of it is that on average the stock market goes up, so even if you are randomly buying stocks you'll come out ahead over the long run as long as fees

A big part of it is that on average the stock market goes up, so even if you are randomly buying stocks you'll come out ahead over the long run as long as fees don't eat it up. Another significant portion is that they have access to a lot of information you don't, and are able to buy stocks and derivatives that you cannot. This comes as a benefit of their size.