Here's why:

* Carbonization: When iron is combined with carbon, it undergoes a process called carburization. This means carbon atoms are introduced into the iron's crystal structure.

* Cast Iron: The resulting material, which is strong and durable, is known as cast iron because it's typically made by melting iron and pouring it into molds (casting).

While the term "carbonized iron" isn't incorrect, it's not as commonly used as "cast iron".