Are you sure about that? It doesn't tie up with my experience. It's true that the leaning forward position on some bikes puts a lot of weight through your arms, but that's riding position - nothing to do with the number of wheels. You can get bicycles with upright riding positions and no weight supported by arms. The steering forces on a bicycle are much less than a tricycle because you have to do work to change direction on trike or quad, but not on a bicycle, where you do much less work to lean the bike. Bikes can steer themselves. Trikes can't.
In the early 70's, when I drove a sidecar outfit, I had chest and arm muscles like Arnold Schwartzenegger. The force needed to get one round a roundabout was massive.