Why do rocket engines carry their own oxidizer for combustion?

Rocket engines carry their own oxidizer for combustion primarily to enable operation above the atmosphere. In space, there is no atmospheric oxygen available, meaning that traditional combustion processes, which rely on atmospheric air as an oxidizer, would not be effective. By carrying an oxidizer, rockets are able to sustain combustion in the vacuum of space, which is critical for powering the rocket and achieving the necessary thrust to propel it beyond the Earth's atmosphere and into space.

While other factors like fuel efficiency and thrust enhancement can be important in rocket design, they do not directly answer the fundamental necessity of having an oxidizer in a non-atmospheric environment. The specific requirement for combustion in space uniquely highlights the importance of an onboard oxidizer in rocket propulsion systems.