Because the control doesn't really have that property. The ToolTip
object causes the IDE to add the ToolTip property in the Properties
window to the other objects in the class.
At runtime, you call the ToolTip objects's SetToolTip method. For
example, if your ToolTip object is called ToolTip1:
ToolTip1.SetToolTip(refToControl, "some text")
Set a tooltip at design time and look in the .designer.vb file for an
example of how it is done.