When you think Facebook/Instagram, you think "Damn, those guys got lucky as hell". When you think automated trading, you think, "Hey, it can't be that hard", and start firing up your IDE and rolling out code to talk to an easily provisioned API.
Sure, it may take months to lose your shirt selling a photo service to Face/Goog/Apple. You can lose everything overnight with automated trading.
My father used to trade commodities for a living in the pit at the CME many moons ago, and when I was growing up I would be his technical side when he was trading out of our suburban Chicago home (setting up FM receiver/satellite dish/etc for real time quote data, staying up late nights with him running through trading scenarios in Tradestation on Win3.1 with data downloaded in bulk from Knight Ridder, and so forth).
Something very important I learned from him was: "The market can stay irrational longer than you can stay solvent." With a startup, you can hit bottom. In the right market, bottom is much further down than you can ever see.