The traffic problem is not just limited to I-5. Every other freeway or highway near the Seattle area is as bad, if not worse. Seattle has traffic, and it's not going anywhere.
Congestion wastes a tremendous amount of time.
All those people stuck in a traffic jam loss precious minutes of their lives behind the wheel. The loss of time for a family member means less time with the family, or less time making money to support the family. So there are potentially two losses - economic or social. Congestion creates more air pollution. Those people aren't alone in traffic.