Why are the organisms living in the terrestrial-air environment more diverse than the inhabitants of the aquatic environment?

Habitats in the sea, freshwater areas and on land differ sharply in species composition and diversity. Of the approximately 1.5 million known species of macroscopic organisms on Earth, the modern ocean, despite its large area and volume, supports only about 15% of the species, while terrestrial environments account for about 80% of the species, and fresh water for the remaining 5%. Typically, high biodiversity depends on high productivity in the long term. Terrestrial environments, for reasons that remain unclear, tend to be much more productive and cover a larger relative area than marine environments.