In a logical explanation, why is ignorance of the law not a defense?

I ask, because it would seem to me that ignorance of the law is the ideal defense.

if some authority figure wants to punish you for doing something it did not want you to do, if this body is claiming to have the authority to be able to punish you in the name of justice, why is it not also this authorities responsibility to also inform its subjects before hand of what they can and cant do.

is it really possible to say ignorance is not a defense and still claim to stand for fairness and justice?

is it possible to defend the argument that ignorance is not a defense without simply stating that it would be inconvenient for said authority if they had to so?

what possible argument could there be in favor of said law?