Question

Why do we not find chains longer than 6 trophic levels

Answer 1

Because the energy that transfers through the trophic levels is only 10 percent [ 10 percent law ] , 
so while coming to the 6th trophic level, the energy becomes negligible, and hence it's impossible for an organism to live with such a low amount of energy, hence Food chains terminate till the 5th or 4th levels.

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Answer 2

Only 10% is the average value of organic matter that is available at each trophic level and is available to the next .Since ,only so little energy is available to the next level of consumers, food chain generally consists of 3 to 4 trophic levels