Why are mitochondria called the powerhouses of cells?

Because it t akes in nutrients, breaks them down, and creates energy rich molecules for the cell.

The mitochondria is called the powerhouse of the cell because it is responsible for producing most of the cell's energy, or adenosine tri-phosphate (ATP). Cells can produce two ATP by breaking down a glycogen molecule, but the mitochondria is able to produce up to 36 ATP in comparison.