Why do we say photosynthesis is responsible for food production for nearly all forms of life on earth?

Because plants are autotrophs which are consumed by heterotrophs like animals/humans. Humans are omnivores, so humans eat meat like cow or chicken and humans directly eat plants. Both plants and meat are consumed by humans and both of those have consumed or

Done photosynthesis

Photosynthesis allows autotrophs to make their own food, then in turn heterotrophs eat the autotrophs giving them food. It's like a big cycle!