Failures in the former may cost their investors their investment, but it won't cost them more money than they've ever made, or put them into the red so deep as to destroy their whole industry, or bring down the global financial system. While both sides play the odds and spread their bets, the nature of risk and payoff is very different.
Also, startups represent an attempt at creating real wealth where none was before - combining labor, capital, innovation, and raw materials to make products that can be sold for more than the sum of their inputs, or services that save time. Alot of what caused the financial crisis was more akin to gambling, despite protestations of 'financial innovation' to the contrary. Failure in the former is admirable, the latter not so much.