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The oil does not handle the heat very well. It does start a number of chemical reactions making the oil bad over time. Just heating the oil makes it form trans-fat bonds which are unhealthy, but it also makes sot and other compounds which may be poisonous and could cause things like cancer. It is still low amounts and probably harmless but still not the best thing you can use to cook. However oil will also react with oxygen and turn rancid. This is not actually a dangerous thing but does make your food taste bitter. Oil will become rancid whenever it have access to oxygen but the heat makes it go rancid much faster.
You can actually reuse cooking oil a few times, fast food restaurants do it. However there is a limit to how long you can use the oil for before it gets bad. So for home cooking it is usually best to just use the oil once and then dispose of it.
The fluctuations in temperature while being exposed to open air over time can change the conformation of the fat molecules, oxidize them, and otherwise cause rancidity.
Who says we can’t, I keep old frying oil and reuse it. As long as it’s not an animal fat, in which case you have to refrigerate it. However if you’re asking why we don’t use it after it goes dark and burnt it’s because it no longer tastes good, eventually the residue from all the foods cooking in it builds up and makes it taste burned and makes all the other food you cook taste like it.
2 reasons, and different oils perform differently.
The first reason is every time you use the oil, tiny bits of the food fall off into the oil. You can strain and filter a lot of these tiny bits out, but some are too small for that. Over time they build up and can start adding a bad taste to your food.
The second, faster reason is oils that are liquid at room temperature are chemically vulnerable to a reaction called “oxidation” that is behind why tons of things spoil and decay. When the oil gets exposed to oxygen, that reaction can happen, and the by-products of the reaction cause a bad flavor. The big culprits are sunlight, heat, and exposure to air. Heat is obviously part of cooking so we can’t avoid that. We tend to store oil in sealed, dark containers to prevent the other two factors.
So in theory if we could keep the oil in a vacuum, filter it perfectly, and never let it be exposed to oxygen, it’d be usable for a lot longer. In reality, we can’t maintain those kinds of conditions in a kitchen.
Each kind of oil has different chemistry, so some oils go “rancid” this way faster than others.
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