My heard-from-online understanding is that this is a combination of multiple factors:

• planes tend to route over major airports so that there’s always an emergency landing site nearby
• there are restricted airspaces that commercial planes cannot fly through
• the most direct path sometimes isn’t the best path. There are stable wind channels in certain areas of the world and it’s more efficient to ride the wind channel than to fly in a straight line

Same vibe as Cato in the Roman Senate: ml delenda est