[logicalmonster]

A big part of the answer is that many people have a fear of looking stupid or having their ego shattered. You're probably not going to be laughed at for trying to do what others have already tried and that society accepts, but you risk being laughed at for trying some unique new marketing idea you came up with.

Another part of the answer is that very few people are experts at all of the skills needed to start a business: being a domain expert in some field, understanding technology, understanding business and money, understanding people, being good at communicating, understanding design, being a good writer, etc. If you're smart, you know that you don't know everything, and you're willing to hear out the advice that others provide.