Because that wouldn't solve the problem for clones of other sites, or clones in other languages. And the Stack Overflow cloners could just make other websites. That's why a primary instinct in search quality is to look for an algorithmic solution that goes to the root of the problem. That approach works across different languages, sites, and if someone makes new sites.

To be clear: the webspam team does reserve the right to take manual action to correct spam problems, and we do. That not only helps Google be responsive, it also improves our algorithms because we get use that data to train better algorithms. With Stack Overflow, I especially wanted to see Google tackle this instance with algorithms first.