A design can start in Photoshop (or Sketch, more likely), but the iterative process of doing high-fidelity prototypes and getting feedback is how good design is done. We are also talking about interaction design here. It's not how it looks, but how it works and feels. That cannot be accurately modeled with a PSD file.
Our design process starts with hand-drawn sketches or paper prototyping and goes up from there in fidelity. You cannot work in an Agile environment with PSD-drive development.
My team has designers, developers, usability experts and people with a combo of these skills. No one is allowed to work in a silo.