It's a slightly older form of the term. "Space" is just space, it's everywhere. Your car has space inside it, so does your backpack, so does your living room, it's all just ordinary space. "Outer" space has specifically referred to the space above Earth's atmosphere, to differentiate it from more Earthly kinds of spaces. Over time outer space has become less exotic and so it tends to just be called "space" colloquially these days, with the meaning understood based on context.