* Higher metabolic rate: Plants, especially those undergoing rapid growth, have a higher metabolic rate than animals. This means they produce more reactive oxygen species (ROS) as byproducts of metabolism.

* Photosynthesis: Photosynthesis, the process by which plants convert light energy into chemical energy, generates a significant amount of ROS. This necessitates a robust defense against oxidative stress.

* Catalase as a key defense: Catalase is a crucial enzyme for breaking down hydrogen peroxide (H₂O₂), a toxic ROS. Plants, with their higher ROS production, need more catalase to neutralize these harmful molecules.

In summary: Plants have more catalase because they generate more reactive oxygen species due to their higher metabolic rate and the process of photosynthesis.