Why do food chains only have 4 trophic levels?

There is only 10% flow of energy from one trophic level to the next higher level. The loss of energy at each step is so great that very little usable remains after four or five trophic levels. Hence only 4 to 5 trophic levels are present in each food chain.

Further Readings – 

1. Energy Flow in Ecosystem
2. Biodiversity – Species, Genetics & Ecosystem