Marketing certainly plays a role in the decision, but it is mainly based on the engineering factor of safety. For Audi, building a vehicle that will last 300,000KM has primacy over squeezing every possible bit of performance out of the engine. Engineers also aren't afforded the time and resources to do real-World testing of an engine as thoroughly as thousands of enthusiasts do. Over the years, through many people's experiences, we start to determine what the real-World limits of the parts are, and mod our vehicles accordingly.