What makes you think this problem isn't being tackled? Every year Twitter and Facebook talk about how they detected and removed XXX million bots from their platform. For example, the spam problem has been rapidly iterated on [1] to little fanfare. The abuse, T&S and security teams at these companies probably cost on the order of hundreds of millions of $, how is that consistent with your framing that the problem is being ignored?

You can't simply throw machines and manpower at a problem.

[0] https://www.statista.com/statistics/420400/spam-email-traffi...