Because the shape and construction of the spacecraft are based on ordinary ships. China attaches great importance to appearance when choosing names. A flying ship is naturally called a spaceship. In foreign countries, they don't distinguish between these. If they travel on water, they are called boats, and if they travel in the air, they are also called boats. Moreover, foreigners have always called spacecraft something like "space vehicles." Foreigners have no concept of ships. Their concept of airplanes, cars, ships and spaceships is all "containers". For example, a container that travels on land is called a car, a container that travels in the air is called a plane, and a container that travels on water or in space is called a ship. Therefore, based on the fact that spaceships and ships are the same concept, their pilots are naturally called captains, and their logs are also called voyage logs. Foreigners are not as clear about these manned containers as spaceships, cars, and ships. It's just that the Chinese call them that, so that's what they call them in translation.