It is as if you mean its there is no difference between tests and step definitions, Step definitions work with their corresponding feature files, then tests cannot work with feature files, cucumber wont see them because there is no their corresponding step definition, cucumber can only watch feature files having step definitions following gherkin scenarios by default.
May be your point clears that we cannot write other step definition classes if we already have their written tests classes because this becomes duplicate, i dont know if the point is clear to us , Feel free to throw more light