This is because the engine cannot run without oil in the same way that the body cannot function without blood. Apart from the ability to conduct some heat away, there is little similarity between oil and blood though. It is not an easy job because the oil has to remain stable and consistent in viscosity and lubricity despite the soaring temperatures in an engine. In temperate countries where temperature go in the other direction, it must also retain the same properties especially during cold starts. Generally, the quality of an oil depends on how well it can withstand the high temperatures in an engine.
High heat is one of the elements that breaks down an oil over time.
Contamination is another element that causes the oil to deteriorate in performance. Most of this come from the combustion process in the form of unburned fuel or by - products such as water. This leads to sludge and foam which interface with oil's lubricating function. The premium range of mineral oils are usually well fortified with additives to withstand these conditions in the engine so that they can perform at an optimum level for a longer time span. If better performing oils are wanted, there is the semi - synthetic range.