Why are there seldom more that five trophic levels in a food chain?

In a food chain e.g, the energy comes from the sun which reaches the grass (producer), which is eaten by the rabbit (herbivore/prey) which eats some of the energy. The fox (predator/carnivore) eats the rabbit which the fox eats the remaining energy from the rabbit, as you can see there has been energy consumed and there is not enough energy to be consumedd further.