Power companies are a classic example of a natural monopoly because they require a ton of extremely expensive physical infrastructure to connect every house to the grid that would be wasteful to duplicate for every competitor.

The whole point of airplanes is that they require no physical infrastructure between point A and point B.

You can have competing power companies generating the power if the grid is owned by the state (or a regulated monopoly). Coincidentally, that is a good mental model for airlines because airports are often state-owned or if not are highly regulated.