It's the human

You can let users choose within certain limits. Others use tri/quadri/pentagrams. I used to have "tarw" when I was a student for example. I've also had 2 different numeric accounts as a student. Nobody complained, the account was there for the whole year, plenty of time to remind it. Also for a very long time you were limited to 8 chars. Many valid names wouldn't fit so that was another good justification to use a short name different from your real one. Actually it's only at home and on my work laptop that I'm using my first name since I'm not creative and it's short. People I know with long or composed names (for example "jean-françois") just use a short variation such as "jf" or "jeff" to avoid conflicts. On the few machines I am/have been managing with up to ~20 persons, often short names are used, with roughly 50% matching the user's first name, indicating that it's more like "I don't know what to use" than "I want my name".

BTW, just look here on LWN: most logins are short (including yours which is among the shortest). When I registered I tried "willy" and it was already taken by Matthew so I switched back to something simple. In any case I needed to note it somewhere, so I could have used anything else, like people have on gmail for example.