Why can't plants live without chloroplasts?

Answer 1

There are some plants that have foresaken the use of chlorplasts by resorting to a life as a decomposure, that is living off of the decaying remains of other living things. One such a plant is 'Dead Man's fingers'. Predacious plants (Venus Fly-trap, Sundew, Bladder-wort, Pitcher Plant) could conceivably do without sunlight as an energy source and take to living solely off of the prey they catch. However none of them have abandoned their photosynthesis just yet - after all if the energy is there why not take advantage of it. Other energy source a plant could conceivably abandon light for would be heat and chemical sources - however these are already heavily exploited by bacteria and Archea and a plant would be hard pressed to invaded into realms already claimed by the worlds most efficent organisms.

Answer 2

That's their energy source.