sometimes they just don't care enough to do it and figure a mod will take care of the problem (if they even give it a second thought). I don't know how big of a problem this is overall, but I wouldn't worry too much about it, OP. Just keep reporting any violations and you've done your part.
Mistakes or lapse of concentration also happen. I know I've forgotten to delete old bumps, and have gone back to do it after either remembering or seeing it again. So, while old bumps aren't necessarily a massive deal, its just general etiquette as it keeps the forum clean, and therefore easier to read.