Question

why are Corresponding angles always congruent? - Why are Alternate exterior angles always congruent?
why aren't Alternate interior angles supplementary?

Answer 1

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.