A) There are plenty of people already tweaking hn in the form of "add ons" like hnnotify.com. That piece already exists and without being the kind of problem that this approach would become.
B) People seem to routinely miss that hn is part of the yc business process. (I honestly don't get how anyone can miss that but posts like this one clearly do. I was a homemaker for eons and I get it.) You know, that is just slightly relevant to how and why things get done the way they are (aka central to the decision making process). This doesn't make good business sense, for reasons other more informed members have already covered.