In many cases it doesn't make sense. For instance, what if your protocol contains a static member?

    protocol P {
        static var x: Int { get }
    }

Here `let x = P.x` doesn't make any sense, because it's only the type implementing the protocol which has that member. However this would be possible:

    func f() -> some P { ... }
    let p = f()
    let x = type(of:p).x

because in this case p has a concrete type, we just don't know what it is.