Physicists use classical mechanics instead of quantum mechanics to describe things like orbits at large distances from Earth or stars because classical mechanics provides an accurate and practical description for macroscopic objects moving at relatively low speeds.
Classical mechanics, based on Newton's laws of motion, works well for objects with large masses and speeds much slower than the speed of light. It describes the motion of objects in terms of position, velocity, acceleration, and forces acting upon them. Classical mechanics is deterministic, meaning that given the initial conditions of a system, the future behavior can be precisely predicted.
On the other hand, quantum mechanics is a more fundamental theory that describes the behavior of particles at the microscopic scale, such as atoms and subatomic particles. It introduces the concept of wave-particle duality, probabilistic behavior, and uncertainty. Quantum mechanics is non-deterministic, meaning that it provides probabilities for the outcomes of measurements rather than definite predictions.
While quantum mechanics is successful in describing the behavior of microscopic particles, it becomes unnecessary and overly complex when applied to macroscopic objects like planets, stars, and satellites. The quantum effects at those scales are negligible and can be approximated by classical mechanics with high precision. Additionally, the computational complexity of solving quantum mechanical equations for large-scale systems makes classical mechanics a more practical and efficient tool for describing orbits and other macroscopic phenomena.
In summary, classical mechanics is used to describe orbits and other macroscopic phenomena because it provides accurate results and is computationally simpler and more practical for such systems. Quantum mechanics is reserved for the microscopic world where its probabilistic and wave-particle nature become significant.