I forgot to add a second reason why there are few innovations in photosynthesis. In most ecosystems, photosynthesis is not the limiting factor. Other things like water, space, mineral nutrients and predation are more important, so there is no evolutionary pressure for photosynthesis to improve.

But in agriculture we solve those problems - we provide space, water and nutrients, and we keep pests and competition away. Then photosynthesis becomes the limiting factor.