Here's why:
* Verbs describe actions or states of being. "Speak" describes the action of using words to communicate.
* Nouns name people, places, things, or ideas.
While "speak" isn't a noun on its own, you can use related words that are nouns:
* Speech refers to the act of speaking, or a formal talk.
* Speaker refers to a person who speaks.
Let me know if you have any other word questions!