Advertisers are afraid of being mentioned in some viral tweet like: "Coca-cola is advertising on the website next to a comment suggesting to kill all black people. They are racist! Let's boycott Coca-cola!"

And Google has a reputation to maintain: they protect advertisers from situations like described above.

Maybe it is not a real problem for advertisers, to be advertised on the wrong website, I don't know. But I understand they don't want to have that risk for virtually no profit.