The issue is less about detection and more about the sheer volume of content. Even if your false positives and negative rates are low the numbers are huge. Which is why everyone ends up hiring more and more people as content volume increases.

What has changed over the years is - the cost to spam the entire world with zillion messages a second is tending to ZERO.

While this is an achievement it's also a disaster cause it's akin to everyone having an almost free high power broadcast transmitter sitting on their roof.

We regulate and auction radio spectrum and don't allow everyone on the planet to broadcast simultaneously on what ever frequency they want. Cause the noise would make signal drop to 0. But on the internet it's allowed and it's almost free.