AIs scale faster. Just one instance can do the work of billions of people, and have its owner profit off it.

How can the owner profit off something that is "free" to make for anyone? Mass spamming is hard to achieve with current day social networks.

Mass spamming is the very raison d'etre for social network companies. It goes by different names though - "sponsored content", "fanpages", "influencers", etc.

The spam did not disappear with people moving away from e-mail. On the contrary, it's just been given legitimacy by the platform owners - who all happen to make money from spam.