I assume it's because large investors move huge amounts of money. When they trade, they cause ripples all over the market. When we trade, the price barely changes.
That is an advantage for the little guy though. If I sell 10 shares when the price is $100, then I'll get $100/share. A huge investor selling 1,000,000 of the same shares might move the price to $98 and average $99/share. Obviously these are made up numbers, and you don't sell a million shares by using just one market order, but the principle is the same. Large investors do have advantages, like better information.
Where do they get better information?
A CEO will answer Blackrock's phone call, not yours. Hedge funds can afford to hire analysts and buy information because they have many orders of magnitude more under management than you or me.
It's not really about where, it's that they do it at all. Are you really going to spend 100 hours researching a company before you make a $1000 investment? If you are going to spend millions dollars on a single company long term you're going to spend hundreds of hours on research before you close the deal.