When you look directly into a lens, your eye may focus on the lens elements and aperture blades themselves, not just on the image formed by the lens. In that situation, the aperture can appear to block part of what you see, so it looks like the image is being cropped.

Through an optical viewfinder, you are not really looking at the lens itself. You are looking at the image the lens projects onto the focusing screen (ground glass). The aperture is far out of focus relative to that screen, so its main visible effect is to reduce the amount of light reaching the image. That makes the view darker and changes depth of field, but it does not crop the framed image.

So the difference is: looking through the lens directly lets you see the aperture mechanism as an object, while looking through the viewfinder shows the image formed by the lens.