I get that it's a black box. After all, nobody wants to tell the spammers what the ingredients of their secret anti-spam sauce are.

The problem is that it's obviously not working; the amount of false positives are staggering and from what I've heard they're still letting a lot of actual spam through.

But, as the sibling comment mentioned, there's little to no incentive for the freemail giants to improve the situation when they're themselves offering paid services with better spam filtering.