As far as I understand, all the energy that living beings rely on comes from the sun. It's processed by plants in photosynthesis. These plants are consumed by herbivores, which in turn are consumed by carnivores. Energy trickles down that way, with photosynthesis being the crucial basis for all the other layers.
A couple of days ago I was wondering, how come animals don't do photosynthesis? This story talks about a few exotic animals that can do photosynthesis, but they seem to be an edge case.
It seems like there's a rule of thumb: Iff a creature can do photosynthesis, it can't move. Why is this so? Wouldn't it be beneficial for a herbivore-like mammal to be able to create sugars directly from sunlight? They wouldn't have to graze at all, just find water and sunlight. It's possible they'd have an easier time avoiding predators and expanding into more territory.
Can you think of a good reason why such creatures never became mainstream?