As somebody who runs their own mail server, I think it's also just a problem of time and relative importance. The full-time job of a spammer is to spam, while I adjust my anti-spam solution only when the spam gets bad enough to prompt action in my spare time.

At this point I'd love to turn running my mail server over to a paid service that I trusted, because they could have full-time people dealing with spam proactively. But Google's free service means a lot of people no longer pay, so the market seems to be very limited.