I would imagine the issue is certainly because they can't. What is hateful to you is charming and encouraging to someone else. Social norms and cultural differences are gigantic. Look at the recent controversy with the conservative guy on YouTube who referred to a reported from Vox as their 'queer Latino reporter' and it was seen as hate speech... despite the Vox reporter openly and frequently labelling themselves as Voxs queer Latino reporter. How is a computer supposed to interpret that? How is it supposed to know that when person A says something and when person B says the exact same words, referring to the exact same subject, that the greater context of the speakers background political affiliations and those of their audience actually determine the 'meaning' behind the statement, not the statement itself?

This is not an easy problem, and it does no one any good to pretend that it is. Tackling the issue also requires those considering it to consider other social situations. Is someone supporting equal treatment of women in Saudia Arabia practicing hate speech against the conservative ruling party? If we'd had systems that let us actively regulate speech in the way we can now, would it have been appropriate to block Martin Luther King Jr. because his message was growing civil disobedience and causing families to bicker over race politics? Why are we so damn certain that any argument today will necessarily be decided by a regression rather than a wider acceptance of more progress? Change in human societies is always ugly, always comes at the cost of pain and strife, and on the balance has usually moved us in a forward direction. I can't say the same for censorship. Censorship makes impossible any forward movement, and only serves to leave regressive mindsets to fester and make-believe that they have more support than they actually do.