Wind, especially, is an important point that nobody mentioned earlier. The jetstream in the northern hemisphere blows from the west. Eastbound planes want to be in it. Westbound planes want to fly above or below it, since the increase in fuel consumption from flying at a non-optimal altitude is smaller than the increase from flying a longer distance through the air. If you check schedules for longer east-west routes in both directions, you'll typically find that the eastbound flight is noticeably shorter.