Here's why:

* Light-dependent reactions are the first stage of photosynthesis and occur in the thylakoid membranes of chloroplasts.

* Their primary function is to convert light energy into chemical energy in the form of ATP and NADPH.

* Carbon dioxide, on the other hand, is used in the light-independent reactions (Calvin cycle), the second stage of photosynthesis.

Here's a simplified breakdown:

1. Light-dependent reactions:

- Capture light energy

- Produce ATP and NADPH

- Release oxygen as a byproduct

2. Light-independent reactions (Calvin cycle):

- Use ATP and NADPH from the light-dependent reactions

- Fix carbon dioxide from the atmosphere

- Produce glucose (sugar)

So, while carbon dioxide is crucial for photosynthesis, it plays no direct role in the light-dependent reactions. It's the light-independent reactions where carbon dioxide is incorporated into organic molecules.