Additional costs for freight and duty are incurred. Made in USA products tend to command a premium at retail, both in the US and internationally. And consistent quality control is harder to maintain. Maintaining quality is a large factor in the long-term success of a product. No matter how good the idea, if the product quality does not meet a customer's needs or expectations, the product will die.
Quality begins with design.
A good manufacturing design allows for some errors and reduces the likelihood of other errors occurring in the first place. Parts that can tolerate errors also cost less than parts that need to be perfect. For example, automated manufacturing machines such as plastic injection molds and metal stamping dies will produce parts of varying thickness with varying surface properties depending on how fast the machines are run. Running the machines faster means that the parts cost less. Likewise, if your product is easy to assemble, the opportunity for error is reduced and labor costs are lower.