Here's why:

* Mitochondria are the powerhouses of the cell: They are responsible for cellular respiration, a process that breaks down glucose (sugar) to produce energy.

* ATP is the primary energy currency of cells: It's a molecule that stores and releases energy for cellular processes.

* Mitochondria use the energy from glucose to create ATP: Through a series of chemical reactions, they convert the energy stored in glucose into a form that the cell can easily use.

So, while mitochondria don't directly produce energy, they are the site of ATP synthesis, making ATP the energy form they produce.