Here's why:

* Binary fission is a form of asexual reproduction primarily used by single-celled organisms like bacteria and some protists. In binary fission, a single cell replicates its DNA and then divides into two identical daughter cells.

* Multicellular organisms use more complex methods of reproduction, often involving sexual processes like the fusion of gametes (sperm and egg) to create offspring.

Here are some examples of reproductive methods in multicellular organisms:

* Sexual reproduction: Involves the combination of genetic material from two parents.

* Budding: A new organism develops from an outgrowth or bud on the parent organism.

* Fragmentation: A parent organism breaks into fragments, each of which can develop into a new organism.

Let me know if you have any more questions!