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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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