How come only a handful of animals can do photosynthesis?
It is all about energy.
It is estimated to take 5000 square meters of plants to feed the average person in the developed world, while the same source gives the lowest possible for a human under ideal conditions at 700 square meters.  In comparison, the average person covers less than 1 square meter.  While there is a lot of waste (animals are able to eat only a few percent of the energy the plant produces), the total energy available from photosynthesis is still orders of magnitude smaller than daily requirements for a typical animal.
Even in the case of the slug you linked (which you would expect would need about as little energy as possible for a mobile animal), it seems they obtain little or none of their actual energy from photosynthesis.
Hence you see photosynthesis restricted to life that has extraordinarily low energy requirements, which typically means being stationary (or moving very, very slowly as in the case some plants).  As soon as you adapt to a mobile, higher energy niche, you need access to a lot more energy than photosynthesis can provide.