Because it's a family honour, as opposed to a personal honour. So, it would contradict the democratic principles of individualism (which requires that people be assessed based
exclusively on their own personal merits). In practice, of course, even in a democracy we assess people based on not only what they do but also who they are, because it makes perfect sense to do so - but nonetheless, it's politically incorrect, so a government would never do anything like that.
(and also, in a monarchy, if you tried to change your surname to a noble surname, you probably wouldn't be permitted to - so, having a special surname is meaningful. In a democracy, there are generally no restrictions on what you can change your surname to, so... well, if the rest of the country can already add "Emden" to their names, it's hardly a great honour for you to be given the non-exclusive "right" to do so
)