Why is the highway designed with left lane for entering and right lane for exiting?

[WalterMarie]

On roads with two or more motor vehicle lanes in the same direction, the left lane is designated as the fast lane, while the right lane serves as the slow lane. Vehicles traveling in the fast lane should maintain the speed specified for that lane. If a vehicle cannot reach the designated speed of the fast lane, it should move to the slow lane. Additional information: Motorcycles should travel in the far-right lane. When traffic signs indicate speed limits, drivers must adhere to the posted speeds. Vehicles in the slow lane may use the fast lane temporarily when overtaking the vehicle in front. On roads with two or more motor vehicle lanes in the same direction, changing lanes must not disrupt the normal flow of traffic in the adjacent lanes.