Does anyone know why there isn't a parameterless constructor for the
String type?

Given that you can...

Dim s as New String("Hello")       ' inefficient, don't do this

I guess I would have expected...

Dim x as New String()

to give me x pointing the an empty string.
For what it's worth, the reason I came across this was because I was
trying to do this in a generic-based class...

Class XYZ(Of T)
  Public Sub New()
     If Not GetType(T).IsValueType Then
        Dim ci As ConstructorInfo =
GetType(T).GetConstructor(System.Type.EmptyTypes)
        Dim obj As T = DirectCast(ci.Invoke(Nothing), T)
        Me.SetValue(obj)
     End If
  End Sub

And when I instantiated one of these of type String, I got the famous
NullReferenceException because ci remained Nothing. The reason: String
has no constructor. I've sinced created a new exception for this class
to throw in such a case, but I still found it odd about String not
having a constructor without any arguments. I'm sure there's probably
a reason; I'm just not seeing it.