It's a social norm. For most people, it's the most "correct" life to wake up early in the morning, have a job with hours somewhere around nine to five, come home around when the sun sets, sleep, and repeat. Unless one has family in medical or restaurant-based fields--where different sleep hours are almost a necessity for dealing with late or changing shifts--it's hard to understand anything healthy about operating later in the day. The thought seems to be that one is most productive by being up and early to do...well, things.
It works if one is trying to network/job hunt for a better living situation that involves a nine-to-five-type gig, but as you know, if you're not doing that, there's no point to keeping such a militarily regimented schedule. My thought is that it's most just a problem that comes from living with family. It's still a bitch of a task to accomplish these days, but there's nothing like having greater freedom and control over one's life, space, and time.
