In a startup, an entrepreneur is starting with a blank sheet of paper. They have only their passion and their team to go with. There is no legacy business, no install base, no reputation risk. They are free to test and iterate rapidly without a lot of resistance.
In an existing organization, there is already a successful legacy business that is repeatable and predictable. There are processes, there is an ingrained culture, there are customers who need to be catered to. A corporate entrepreneur looking to start a new venture has to take stock of all of the above and figure out how to leverage the assets and circumvent obstacles.