Because the properties of a 'Describe' object are not a fixed list as they are for the other object types in your autocomplete examples. The list of properties depends on the type of object passed to
arcpy.Describe(). As the Esri documentation puts it:
The Describe function returns a Describe object, with multiple
properties, such as data type, fields, indexes, and many others. Its
properties are dynamic, meaning that depending on what data type is
described, different describe properties will be available for use.
( http://desktop.arcgis.com/en/arcmap/10.4/analyze/arcpy-functions/describe.htm )
Update: The Python editor gets its autocomplete list from an object's type rather than from an instance of an object.
Although there may be ways to query an object instance for all of its available attributes, this is not how the editor works (and such methods may or may not be appropriate for this context).
For some more discussion on accessing all the attributes of an object instance, see