If it looks and smells good, why is it still there a week later?
Well, there's lots of reasons for that. Mind you, we don't have leftovers for a week on a regular basis, but it does happen occasionally. I think it's pretty obvious when food starts to spoil, so I'm not going by an amount of time. I'll toss something after a day or two if it doesn't look and/or smell and/or feel right.
Reasons why food stays in the fridge for a while:
1) With 2 adults and 3 kids, not everyone always has a full appetite. Sometimes my wife had a late lunch or there was food at work and came home not as hungry and skipped dinner. Or I did the same. Or my son had soccer practice and now isn't that hungry. The portion I was going to cook doesn't change.
2) I planned on leftovers and then we decided to go out to eat. And then brought home more leftovers. And those leftovers were better eaten the next day.
3) I bought stuff to cook over the next few days or took something out of the freezer and I need to make it before it spoils prior to being cooked. Next thing I know, I have a couple days worth of leftovers.
4) We go out of town for a couple days or a few days. I'm not tossing food just because I leave. If it's still good when I come back, then great. If not, I'll toss it then.
5) I make a large amount of something (chili, soup, Thanksgiving, etc).
5) Combine any number of the above and next thing you know you've got things in the fridge for a week. If I have something I know can last that long, then I'll eat things in the order that they are likely to not last
Again, doesn't happen often but I don't think I've ever counted days. It's either good or it's not.