These products tend to cover wide range of requirements - for someone headphones plugged into the "green" audio out is all that's needed. Someone else might have "fancy" surround speaker system with three separate connectors. Yet someone else might want to use SPDIF (optical or coax). And there are people who upgrade their speaker systems and change connectors.
SPDIF is digital audio interface. It does not add more than a dollar or two to the total cost of the motherboard. And then - no one forces you to buy anything.