Human beings don't directly photosynthesize for clear reasons. Solar power isn't available constantly and is too diffuse to power us in real-time. Instead, we eat plants (or animals, which is just another layer of indirection) that have photosynthesized over time and stored up that energy - we then convert that energy so we can metabolize it as needed.

The same thing is happening with petroleum. It's denser and more flexible than the sun. These are real constraints that have led us to use these sources of energy and prioritize them. They're not insurmountable - I think as prices fall we'll see more research into storage, I believe panel efficiency is going up - but the path of least resistance is a powerful force.