China Dominates 44% of Visible Fishing Activity Worldwide

Short answer is energy efficiency.

All energy on earth derives from 1) the sun or 2) geothermal

Energy is lost as you move further from those sources. Plants converting sunlight directly to usable energy are more efficient than a higher order animal eating another animal that ate another animal that ate a plant.

Now nature normally balances this hierarchy in a myriad of ways that you can go read about yourself.

The problem is humans have rapidly expanded and want to consume more than nature can provide and restock. We have exceeded the capacity for people surviving off animal products.

Attempts to produce more animal products is one of the major drivers of climate change, alongside things like concrete.

Short answer indeed. Cellulose, which is most of the plant mass, has a ton of energy stored in it but we can't access that while many animals we eat can.