Standard Colors

Why are Standard Colors more readily available than custom colors? 

According to AEP Span: 

Metal roofing and siding products are manufactured from long strips of painted steel that are tightly wound into rolls, called coils. All of the coils from which panels are produced are prefinished and stocked on the production floor, slit to the width necessary to roll form the profiles we offer. This is necessary to be able to respond to construction schedules without adding to the production time required for the coil-coating, slitting, and coil delivery process to our multiple manufacturing facilities.