Why do we use derating factors when sizing conductors?
The simple answer is that it provides a safe operational limit for specific conductors based upon insulation type, installation method, and temperature (among other conditions).  The National Electrical Code (NEC) gives specific guidance in article 310.  The manufacturer's product data gives a value based on material construction without regard to application.  De-rating is necessary to ensure that no damage occurs to the conductor or insulation.  For example, multiple conductors installed within the same conduit or raceway would typically be rated at a lower ampacity compared to a single conductor of the same cross sectional area installed in free-air.