Here's why:
* Tú is the singular form of the second person pronoun in Spanish, meaning "you."
* Tu amigo is singular, referring to "your friend."
* When you have two or more people (in this case, you and your friend), you need the plural second-person pronoun, which is vosotros.
Therefore, you would say "Vosotros sois amigos." (You two are friends.)