Here's why:

* Algae are simple organisms that don't have the complex structures of plants like roots, stems, and leaves. They lack the need for extensive supporting tissues to maintain their form. This allows them to dedicate more of their biomass to photosynthetic tissues, which are directly involved in capturing energy from the sun.

* Plants, on the other hand, need significant supportive tissues (wood, bark, etc.) to provide structure and transport water and nutrients. This means they have a lower ratio of photosynthetic tissue to supportive tissue compared to algae.

Therefore, algae have a higher proportion of biomass dedicated to photosynthesis, making them more efficient at converting sunlight into energy.