Why is it Difficult to Manufacture Parabolic Mirrors?

[bomba923]

Spherical mirrors, no problem, are easier to manufacture. But

*WHY*

is it difficult to manufacture/produce a parabolic mirror?

e.g., something like parabolic reflector with a focus about a meter from the vertex,
and about half a meter in diameter?

 

[pallidin]

Spherical mirrors have a constant curvature. During manufacture and finished polishing, it makes things simple.
Parabolic mirrors do not have a constant curvature, and though this differential curvature is easy to mathematically define, the manufacture and polishing can be tricky and expensive(depending on the "tolerance" of perfection desired)