Question

Why do many food chains only contain 3-4 links?

Answer 1

That's how the food chain works. There isn't much to it.

Answer 2

because of Ten Percent Law. Energy reduces by 10% when it goes to next Trophic level in food chain. For Example:

Grass----Grasshopper----Rabit-----Sanke---Eagle

Eagle will have the least energy. So that is why food chains have only 3-4 Steps.!!