I think this phenomenon is caused by the culture of fear among most employees. Screwing up badly will get you fired, the greatest fear of all. Most employees have heavy mortgages and children, and require a consistent and predictable income. (This state of things is a problem in itself, but that's another topic).

The big issue is that the level of fear increases as you move up the command structure.

Let's assume one of your startups has crashed and burned. Something really spectacular must have happened if this makes potential customers of your next startup spurn you. Taking risks may have consequences, but they are temporary. But if you are a high-level manager for a large company, spectacular failure will make you unemployable, or in the best case push you several steps down the ladder - steps which took years of boredom and politics to climb, and which you will never get back. Failure is punished disproportionately, success is rarely compensated. Conservatism is the correct choice for each individual actor.

We all know bureaucracy, culture and politics - existing companies won't change in this area.