What is an explanation of why producers are always found at the lowest trophic level?

Producers are autotrophs and do not depend on other trophic level organisms for their food.

Explanation:

Producers are autotrophs and produce their food from sunlight through photosynthesis. They convert sunlight into chemical energy which is transferred to other trophic levels such as primary consumers, secondary consumers, and others if present.

All the trophic levels other than producers are depending on the producers or other trophic levels for their food.

Thus, producers are always found at the lowest trophic level.