Why not just a drag and drop functionality? Rather than adding a field to a layout, layout to a product, etc.....
Thank you for your idea. The current design of adding fields to a layout then assigning the layout to a product allows for layouts to be built once, then used across many products. This provides the additional benefit of being able to apply a layout to a product line and have it inherited to all products within the product line.
Given the benefits above, while we may make updates overtime to streamline the way custom fields are created we are unlikely to change the design of using custom field layouts at this time. We hope you can understand.