Because there are sociopolitical actors who can extract direct benefit from OTHER people believing misinformation to their detriment. You are omitting this motivation, and it's a significant factor, worth spending money and energy on if your motivation is to harm a population that you see as your enemy.
This assumes a global marketplace of information propagation. That's the new factor here: the actual motivation isn't all that new.
I wonder how prevalent the will to "harm a population" is. I think it's more about benefitting oneself financially than trying to harm others. Politically speaking yes politicians try to harm (the popularity of) their opponents. But politicians are a very small slice of the population.
This assumes a global marketplace of information propagation. That's the new factor here: the actual motivation isn't all that new.