Why do we not know the full number of species on earth?

The full number of species on Earth remains unknown due to several factors, including the vast diversity of ecosystems, the remote and inaccessible habitats that have yet to be thoroughly studied, and the continuous discovery of new species. Many organisms, particularly invertebrates and microorganisms, are small or cryptic, making them difficult to identify and classify. Additionally, the ongoing loss of biodiversity through habitat destruction and climate change further complicates our understanding, as species may become extinct before they are even documented. As a result, estimates of global biodiversity vary widely, and only a fraction of species have been formally described.