Why do upper-level winds in the middle-latitudes blow from the west?

Answer 1

High altitudes winds at mid-latitudes blow from the west because they are influenced by the earth rotation. But it depends on the scale and time you are talking about.

Answer 2

Upper-level winds in the middle-latitudes blow from the west due to the combination of the Coriolis effect and pressure gradients. The Coriolis effect causes winds to deflect to the right in the Northern Hemisphere, which, combined with pressure gradients created by the temperature difference between the equator and poles, results in westerly winds at the upper levels of the atmosphere.