Here's why:

* Carbon dioxide is the source of carbon: Photosynthesis is the process of converting light energy into chemical energy in the form of sugars. Carbon is the backbone of these sugars, and plants obtain that carbon from carbon dioxide.

* Oxygen is a byproduct: While plants use carbon dioxide, they release oxygen (O2) as a byproduct of photosynthesis.

While plants need oxygen for respiration (like humans), they don't use it for photosynthesis.