Pairing a route like LAX-SEA with SEA-JFK/DCA (which AS does) to route it LAX-SEA-JFK/DCA helps the utilization of the plane.
Pity you didn’t bother to check this out before you asserted it only gets flown on LAX-SEA.
https://www.alaskaair.com/status/7/2021-10-17
https://www.alaskaair.com/status/1/2021-10-17