You have dreamed it up, but who is going to make it real?
You often need more than one person due to various skill requirements, exotic technology used or some other odd reason. I have the right skills and know the right people for the skills I don’t have.
This makes going from an idea to a product you can touch a smooth process you can predict.*
Hardware is hard and due to that prediction is, too. I give liberal estimates to account for that.