Here's why:
* Present simple is used for actions that happen regularly or are generally true.
* Speak is the base form of the verb.
* In the present simple, the base form is used for all persons except the third person singular (he/she/it). For the third person singular, we add "s" to the base form, making it "speaks."
Here are some examples:
* I speak English.
* You speak Spanish.
* He speaks French.
* They speak German.