A sad truth. From my experience, a lot of it comes down to "who looks like they're working more?" The sub-par engineers, god bless 'em, are just clearly going nowhere. The great engineers go home at 4 or 5, knowing they've written some great code and removed a lot of bad code. So who's left working at 7 or 8? The mediocre guys who have to put in more hours to get the same results.

So yeah, eventually the great engineers get tired of playing code janitor and bail.