Here's why:

* Yeast is a fungus. Fungi are heterotrophic organisms, meaning they cannot produce their own food like plants do.

* Photosynthesis requires chlorophyll. Chlorophyll is a pigment that absorbs sunlight and is essential for photosynthesis. Yeast does not have chlorophyll.

* Yeast obtains energy through fermentation. Yeast breaks down sugars (like glucose) into simpler molecules, releasing energy in the process. This is called fermentation.

So, while plants use photosynthesis to convert sunlight into energy, yeast uses a different process, fermentation, to obtain energy from sugars.