Do we really need defamation laws?
Why don't we just teach children to think critically? (What if a significant proportion of parents refuse?)
Are people so deeply irrational that that won't work?

@Hyolobrika absolutely. People will not only instantly believe defamatory lies that confirm their current opinion, suspicion or general worldview, but will also resist any attempts to refute them. A threat of severe consequences and a requirement for official retractions are the only way to keep them at a sane level.

@Hyolobrika and critical thinking 1) is rare and pretty much impossible to "teach" most of society in current cultures and 2) doesn't even work that well for people trained in it, our monkey brains are too susceptible to biases.