"If evolution is a fact, why is it still called a theory?"

First, "evolution" has come to have multiple meanings, including the
fact of evolution (what happened) and the theory of evolution (how
it happened). Using the same word for both makes many people think
that the fact and the theory refer to the same thing, when in fact,
to call the fact a theory is, in this sense, simply a mistake.

Second, no fact can be proved absolutely (except in artificial
universes such as mathematics and the world Creationists want to
live in).  Some people like to recognize this by referring to
facts as theories.