Soupy, not sure if that's one for me.
If it is, the answer is, I really don't know why? I just can't eat/drink things past their sellby date, it's some sort of weird white coat syndrome, I put my faith in that little stamp.
Having said that, I am horrified when I do find something that's off - I took a big gulp of milk from a fresh carton a few years ago, it was curdled and disgusting. I have never slurped from the carton since.
One of my eccentricities I suppose.