Here's a guess. Philosophers will discuss, define, and analyze what evidence is, and what qualifies as evidence in what context. Our task is to develop and describe the nature of evidence. Scientists, ordinarily, run with assumptions about what evidence is, and design and interpret experiments created to answer a question about the empirical world. Philosophy sets the ground rules for what a satisfactory answer would look like; science then goes and looks for answers. We don't deal with evidence because we do the conceptual work that allows us to categorize some observations as good evidence, and others as irrelevant or uninteresting. Just a thought.