Here's why:

* Land Plants: Land plants have vast surface areas covered in leaves, which are specifically designed for capturing sunlight for photosynthesis. They have access to abundant carbon dioxide in the atmosphere.

* Ocean: While phytoplankton in the ocean do perform photosynthesis, they are limited by factors like sunlight penetration in water, nutrient availability, and grazing by animals.

Overall, the sheer amount of land covered by plants and their efficiency at photosynthesis means they contribute significantly more to the global oxygen production than the oceans.