If ignorance of the law is not a defence, how are you supposed to know all the laws?

You'll never know all the laws. Even people that know the law can argue over it's meaning. That's why lawyers are important.

Governments make them freely available to the public. Basically, everything the government produces that affects the public is openly available. You should be able to find you're local and state laws easily inline or at a government office.

When you got a drivers' license you presumably learned the laws regulating driving. All laws that I've seen (and I look them up just for fun) are either common sense, or like driving laws, they apply to specific activities which you could choose to learn about when you decide to take up the activity. Since you could have learned, you are legally (and morally) responsible if you chose not to.

I'm wiring my garage. The NEC (National Electrical Code) is very large, and turgid. It's my choice to plow through the mind melting verbosity rather than hire a subject expert.

Imagine if it was plausible to not know the law. That way people could do something they knew was illegal, and just claim they didn't know. That's why it's not a defence.

Because like the other poster said, its public information to get information about the laws. If you choose to not learn them, then thats your consequence.