A simple answere is energy management:
Photosynthesis does not deliver enough energy for the cell that does photosynthesis to move, its barely enough energy to reproduce self, and it does not have to move (only its childrens have to move away from the parent, generally done by wind).
The sun is not always shining, but storing energy is easier than searching sunnier places, that are plopably out of reach anyways!!!
Animals that eat many cells that do photosynthesis can get enough energy for movement, and they have to move to find enough food because it only grows were they are not eating! They are less dependant on dark or dry periods and more flexible in finding energy sources.
Animals that eat animals can get even more energy and have to move even more and are even more flexible (most can still eat plants, too)...
The suns energy at one place at first gets used by cells that do not move. Than it gets used by slowly moving cells that collect that stored energy from a few places. Than it gets used by faster moving cells that collect the stroed energy from a few slowly moving cells...
Animals do not develop solar collectors because eating plants is just easier and if an animal could do photosynthises its ability to move would be ambiguous like a whales foot.
There are also lithorve bacteria eating rocks and sand, they move really slowly.