Established companies already have customers, product market-fit, brand recognition, revenue and market share which removes a lot of the financial, operating, market, product and technical risk. Startups must achieve all of these key milestones from a cold start. Their startup teams have to create something from absolutely nothing and there is a very long list of tasks to complete for this “something” to happen.
Startup success depends on this huge number of tasks going right (eventually) and a team correctly managing the sequence and risks of these tasks along the journey. To name just a few you will have to innovate, make products, create a business plan, tell a story, secure resources and/or raise money, refine the story and tell it again and again. Perhaps more important you have to build and lead a team, execute, solve problems, get your market timing right, achieve product-market fit, market and sell to customers and make them happy (and in the case of conventional businesses - happy enough to pay you). It’s a long list of milestones and results that require you to carve out a path to success within the limited time you have to achieve it.