Multiple players along the chain could decide to vertically integrate. Although I used the example of high frequency traders, a better question which I should have asked is why hasn't someone dipped their toes into retail and HFT? It seems like someone would want to grab that land for themselves if it were particularly valuable, which the valuation of Robinhood purports. Considering some guys in that industry made this app, I don't think it's too far a stretch of the imagination for someone to believe that an industry player could have made that move.