The professional trader skim the cream by having access to the order book without the 15 minute delay or whatever is used. Not surprising consumers would rather do the exchange off market?
Google/Yahoo give delayed data away to the public for free, but if you're actually trading, you have a broker and you won't wait fifteen minutes for data.
It's true that professional traders have less latency because they spend money on that, so they can act faster. But as a retail investor I really don't care if they pocket about a quarter of a cent per share on each trade I make, I'm happy there's liquidity and trading costs have never been lower for the little guy than in the present era.
For example: https://nasdaqtrader.com/Trader.aspx?id=DPUSdata