Why is the wind direction parallel to the isobar in the upper atmosphere?

the horizontal movement of the atmosphere is called wind, and the direct cause of wind formation is the horizontal pressure gradient force, which is perpendicular to the isobar and points to the low pressure. At high altitude, the wind direction is only affected by the horizontal pressure gradient force and the geostrophic deflection force, and the wind direction is finally parallel to the isobar; Near the ground, the wind direction is also affected by friction, and the wind direction finally intersects the isobar.