Then why is the scientific community not willing to say case closed and assert it as undeniable fact?
They do. They say every day that "this evolved into that". They just don't put the word "fact" in there.
The fact of Evolution is explained by the "theory of Evolution". Neither Evolution, or the theory of gravity, or the theory of nuclear attraction will EVER "graduate" to "fact".
You are confusing the common meaning of the term with the scientific meaning.
Evolution is both a theory AND a fact.