do you need a degree to be a painter? why should you need a degree to be an interior designer?
Because, as OP states in the first half of his post, an interior
designer is more than just decorating.
It is seemingly internal architecture.
I certainly want someone qualified to decide where to have electrical cables and gas supplies and water flow, as well as where to have emergency exits and whatnot. They're not things you can place will-he-nill-he. If done wrong you can cause fatal accidents.
And while earning a degree does show commitment and atleast some level of knowledge in the subject, I would quite like a professional examination for anyone who does this job, in order to be sure that they're up to the task.