When somebody has an idea for a product, the tendency is towards these people being very involved in their production. They like to take the initial concept of a product and refine and develop that concept into a real product. This is basically done because most people like to retain a sense of ownership over their creations, which is understandable, but somewhat limiting.
The drawbacks to somebody who’s come up with an idea attempting to complete the whole design are that it limits the involvement of professional designers. Product designers have a lot of expertise in aesthetics and ergonomics which have a massive impact on how people view and experience a product. Much like with how a chef doesn’t reach a conclusion about a dish they’ve made without the approval of a taster, product designers need to have a few different views to come up with a great product.