You're paying your most expensive employees to build something twice, and initially in a lower-fidelity tool that won't quite mimic the functionality or the look the of the final product. So you end up with stakeholders that don't understand it's just an interactive demo, and start wondering why things don't quite work. That or they buy into the demo, you build the real solution and then they're questioning why it doesn't quite work the same, or looks different, because when does an interactive demo in a mockup tool every really get it 100% right?