Surely the optimal way to proceed when faced with betting on a coin toss is to keep your stake and guarantee you walk away with some money.

Now it may be possible to evaluate the utility of engaging in a start-up and so act rationally in entering in to such an endeavour but I'd warrant that's not how it [ever] happens. Instead it's far more realistic to assume it follows the lottery model: that people's sense of hope, or wishful thinking, makes them choose to play [at lottery, or startups] despite the analysis indicating an alternate choice is most logical. We humans are not creatures of logic.