Why are Corresponding angles always congruent? - Why are Alternate exterior angles always congruent?

Why aren't Alternate interior angles supplementary?

Why are Corresponding angles always congruent? - - > A postulate says this is true when two parallel lines are cut by a transversal.

Why are Alternate exterior angles always congruent? - - > A theorem says this is true when two parallel lines are cut by a transversal.

Why aren't Alternate interior angles supplementary? - - > A theorem says that when two parallel lines are cut by a transversal, then same-side interior angles are supplementary and same-side exterior angles are supplementary.