Here's why:

* Fertilization results in a single cell called a zygote, which contains the complete genetic material from both parents.

* Mitosis is a type of cell division where a single cell divides into two identical daughter cells.

* The zygote undergoes rapid mitotic divisions to create a multicellular organism. This process is called cleavage.

Other important points:

* Meiosis is another type of cell division, but it occurs in the production of gametes (sperm and egg) before fertilization.

* Mitosis continues throughout the development of the organism to produce more cells for growth and repair.

Let me know if you have any other questions!