The answer is easy. Because it’s an artificial way to influence search results – to hide a default site from search results that Google deems to be low quality. A site that has a default wordpress installation – to test plugins and such – isn’t really going for top quality search results.
But, this technique can also be used to deplatform sites that don’t toe the ideological or information that some diptshit engineer at Google has with his or her thumb stuck up her butt.