FAA sets the rules.
They're designed to reduce collisions.
East flies at odd-000 feet
and West flies at even-000 feet up to 29,000, then they alternate every 2,000 feet. With better radar, better computers, and onboard collision avoidance equipment, controllers are able to reduce the vertical separation and accommodate more aircraft They fly at a specific altitude because the airlines save money on fuel if they fly between 33-42,000 feet. The air is thinner here and the plane uses less fuel to fly from one distance to another.