Because they go in the back of the fridge and you forget they exist. That is the whole reason. It is not laziness, it is placement.
Three fixes, in order of how much they help.
- Put leftovers at eye level, at the front. The average household throws away food it fully intended to eat. If you cannot see it, you will not eat it. Give leftovers the best shelf, and put the condiments at the back instead.
- Write the date on it. Not the contents — the date. Masking tape and a marker kept next to the wrap. Without a date you will stand there wondering whether it is Tuesday's or Sunday's, and you will bin it to be safe.
- Decide when you put it away, not when you open the fridge. As you wrap it, say out loud what it becomes: tomorrow's lunch, Thursday's fried rice, or the freezer. Leftovers with a plan get eaten. Leftovers without one become a science experiment.
If you want to know what it is actually costing you, log everything you throw away for two weeks — what it was and roughly what it cost. Two weeks is enough to see the pattern. Most households find one or two repeat offenders, and once you see them you simply stop buying them in that quantity.