aviadornovato and others have suggested that flight levels should automatically change if a route segment in the flight planned route changes direction between easterly and westerly tracks from the last route segment.
Doing this would create substantial extra workload for ATC.
Consider an a/c at FL200 that has to change from an even altitude in a westerly segment, say 350 degrees, at waypoint X to an odd altitude in the succeeding easterly segment, say FL210 at 010 degrees.
An opposite direction a/c approaching X at FL200 tracking 190 degrees (westerly) has to change from even to odd altitude, say FL210, when changing track to 170 degrees (easterly).
ATC has to ensure that both a/c are past each other before clearing them to trade flight levels.
ATC also has to ensure that the climb and descent do not conflict with following a/c. There are some casino dealers who can shuffle a deck with great efficiency, but this is not something ATC would want to do.
Workload is far more manageable with a/c remaining at their cleared altitude.