But what if it doesn't matter if it takes a day or a week? And how is the business person to know? Just because Joe asks for a lot of money, are they supposed to believe that he is good?

I think you are confusing your personal experiences and situations with the situation of some business guy. They are not out to start a software company, where they will manage programmers. They just want product x build.

It is an exchangeable skill, in the same way as there are thousands of mechanics who could fix your car, or thousands of window-makers who could create a window for your house, or thousands of carpenters who can create a cupboard to your specifications.

If you drive your car into some mechanics garage, you are not expecting to manage that mechanic. You just want your car fixed.