Here's why:
* "Tu" is the informal "you". You wouldn't use "tu" with someone you don't know well.
* "Hablamos" is the first person plural (we) form of the verb "hablar" (to speak).
Here are some correct ways to say "We speak Spanish" in Spanish:
* "Nosotros hablamos español." (formal)
* "Hablamos español." (informal, when speaking to someone you know well)
"Tú hablas español." would be the correct way to say "You speak Spanish" (informal).