>I've always wondered why two-weeks notice is so engrained culturally when an employee decides to leave on their own.

I'm torn. On one hand, just not showing up is a pretty crappy thing to do to anyone in any situation. On the other, you're right; a company may be perfectly fine leaving you high and dry without notice. Sometimes people in our position get severance or even a heads up, but often times people don't. It's not fair, but I suppose the reality is that many will want a good recommendation, but no one pegs the boss with a layoff.