Yeah but it's a legal liability problem.

Food isn't instantly poisonous as of the best by date, but producers will have insurance that covers them for medical costs if someone gets sick from their food.

Obviously, that insurer is not going to cover them if the stuff is sold (or distributed) after the best by date has expired.

Suppose your local grocery store starts a "best by" program and gives out the free food which is out of date. Some customer eats a free bread role which is a tiny bit stale but otherwise satisfying... and then eats a dead pigeon the find on the side of the road.

You can probably see the problem.