Many entrepreneurs start a business because of a passion around an idea, product, or service. They usually have great ideas and can connect that with a customer need or problem. I also see that many entrepreneurs are really good at that passion; but may lack some skills or understand of the "running the business" side of the equation.
An example would be if you want to open your own manufacturing business - have you ever worked in a manufacturing business? I highly recommend working in a business that is similar for at least a year so you learn what works well or what doesn't work well on another person's dime. This is especially true when working with high start-up costs or a very limited initial cash flow which cannot tolerate costly mistakes.
Go work for someone or a business you can learn from is my answer :)