From college events, supermarkets, schools....

People don't shop the way they used to, buying perishable items daily in the cities or pulling them out of the garden or off the canning shelf in the country for that day's food. They bring home a week or two worth of perishables and the stuff that can't be frozen often doesn't make it the full week or two. They forget what they bought or what they were going to use it for and that's another contributor.

Most people don't have time to cook everything they buy, extra hours required at work without notice leading to more green slime in the fridge.

Supermarkets won't sell stuff that isn't at the peak of freshness. Few are giving to soup kitchens these days, it's just cheaper to toss it into the dumpster. Food that rots in the field usually does so because the price has fallen so much that it's more useful as fertilizer for the next crop than it is on the market and feeding people. Fast food joints routinely toss everything on the steam table that didn't sell instead of passing it out the back door to the homeless.

Farmer to home kitchen, the whole system favors waste. If this country is serious about cutting waste, it's going to have to subsidize perishables like fruits and vegetables, cut the work week, penalize employers who try to work a few employees to death rather than hire more, raise wages, and encourage micro businesses like greengrocer carts going through neighborhoods to encourage frequent shopping with less waste.

I know I've cut waste in my own kitchen by shopping twice a week, once at the health food store and once at the supermarket. I'm more aware of what I've got and what I need if I don't have to keep the inventory in my head too long.