A rangefinder uses separate windows for viewing and focusing, so moving the main viewfinder directly above the lens wouldn’t eliminate parallax entirely—it would only trade horizontal parallax for vertical parallax. Since some parallax will always remain on a non-TTL system, there isn’t a complete framing advantage to putting it above the lens.

There are also practical reasons for the usual offset placement. Keeping the main finder away from the lens helps avoid the lens obstructing part of the view. And the traditional top-left position has ergonomic benefits: your right eye can get close to the finder without your nose pressing awkwardly into the camera back, and your other eye can stay open more easily to watch the scene around the frame.

So the offset finder is a compromise: parallax can’t be fully removed anyway, and the side position often gives a clearer view and better handling.