The reason is very simple: They rely on Google Safetynet (basically self-diagnosis). And that will immediately tell you off if it notices your device is rooted. And while you can have a lengthy discussion regarding whether this makes your phone less secure or not, this is another simple argument from Google's POV: The device has obviously been tampered with, we don't want to put any resources into covering this case. As far as we are concerned, you shouldn't use our OS like this.