Why does selecting an attribute of an Oracle object require an alias

My GIS software has an Oracle 18c object type called ST_GEOMETRY.

The object type has attributes:

The attributes of the spatial type represent the following
information:

• Entity: The type of geometric feature stored in the spatial column (linestring, multilinestring, multipoint, multipolygon, point, or
  polygon), the value of which is a bit mask derived from the
  st_geom_util stored procedure.
• Numpts: The number of points defining the geometry; for multipart geometries, this includes the separators between each part, one point
  for each separator.
• Minx, miny, maxx, maxy: The spatial envelope of the geometry
• Area: The area of the geometry
• Len: The perimeter length of the geometry
• SRID: Contains the identifier for the geometry that links it to its associated spatial reference (coordinate system) record in the
  ST_Spatial_References table
• Points: Contains the byte stream of the point coordinates that define the geometry

I can select an object's attribute using one of two methods:

1. Use Oracle's TREAT() function.

select 
    treat(shape as sde.st_geometry).numpts as num_points
from 
    a_test_table

NUM_POINTS
----------
        37
       161

2. Or I can simply use an alias:

select 
    a.shape.numpts as num_points
from 
    a_test_table a

NUM_POINTS
----------
        37
       161

Question:

Why is it necessary to use an alias to select an attribute from an Oracle object?

If I were to remove the alias…

select 
    shape.numpts as num_points
from 
    a_test_table

Then I would get an error:

ORA-00904: "SHAPE"."NUMPTS": invalid identifier

Edit:

This appears to apply to the native Oracle SDO_GEOEMTRY methods too.

select 
    a.shape.get_wkt()
from 
    table a