They've done that for other races, and races that players don't have access to like half-races. I imagine the reason is it's not too hard for them to mix and match options, and sometimes they work out wonderfully, but because of the times it doesn't work out wonderfully and the extra information they would have to pass around they don't allow it for players.

Player information gets passed around a lot so if there is too many options it might cause issues (like why crowds in video games tend to be variations of repeats), and certain options, especially in combination with other things like gear, might clip or cause glitches that they don't really want to try and fix or even find out if it works by through extensive QA (like super long hair given to some npcs).

Well the above and they probably want to ensure the unique quality of some of their npcs so it's like "you're Zenos? No I AM ZENOS!" lol.