Here's why:

* Essential for Photosynthesis: RuBisCo is the key enzyme in the Calvin cycle, the light-independent reactions of photosynthesis. It catalyzes the initial step of carbon fixation, where carbon dioxide is incorporated into organic molecules.

* Found in All Photosynthetic Organisms: Since photosynthesis is the basis for life on Earth, RuBisCo is found in all photosynthetic organisms, including plants, algae, and some bacteria.

* High Abundance: Due to its vital role in photosynthesis, RuBisCo is present in massive quantities in photosynthetic organisms, making it the most abundant enzyme on Earth.

While precise estimates vary, it's safe to say that RuBisCo makes up a significant percentage of the total protein content on our planet.