Why are most communities best represented with a food web instead of a food chain?
Because in nature there is not actually such a thing called a food chain. This is just a way to simplify the food web by drawing each line separately.
In nature, each organism depends on several other organisms for food, for example a fox can eat a rabbit, a duck or any other small animal available. The duck can eat grass or insects. The insects the duck eaten by can be of different types, some herbivorous and some carnivorous. And so on, so if we would like to represent what is really happening in the ecosystem, a food chain will not be a good real representing model.