There's no reason "why," in the way you want to think about it.

There are some incredibly complicated bots out there that do various complicated things to acquire accounts and post things automatically. I don't believe humans interact with them in anyway — beyond updating the spam database.

I have one contact form that spammers regularly use to send me single words. "Carss" [sic] was popular last week. Who knows why. They can't put URLs in the contact form at all... that clearly has something to do with it.

Anyway, the reason it seems so stupid is because the spammer likely has no idea they're hitting this site at all ... and they don't even care. Nor would they notice if "we" somehow blocked all the messages in the first place.

One thing that might help a little is if there was no way to post anchor tags or URLs at all unless they were in the proper [href://|lol!]] markup. It's unlikely the botnet operator(s) would go through the trouble of learning that markup since it wouldn't do any good anyway.

We would definitely still get the random word spam from the bots, but it'd be somewhat less common.