Of course there is such thing as junior and senior in deaign. It comes with experience and understanding of process.
What you’re saying is that experience doesn’t matter and that everyone can so the same job. That simply is not how it works.
You have to be able to develop and deliver on a schedule and the only way you can do that is with a clear roadmap and knowing how to direct time and resources over the course of a project. That comes down to judgments that can be expensive in the process.
Resources (time and money) are limited on a design project. A senior designer can more effectively steer the ship. They know the working parts and what it feels like to get it right each day.