It’s a lay over of legacy design. In the era of manual steering even steering a car at low speed is heavy and difficult. A truck or bus with manual steering really required some strength. In a truck or bus at low speed you pretty much pulled on the steering wheel with your entire body not just your arms.
A large horizontal steering gave you some advantages in using your whole body to pull or push on it. Today most buses have power assisted steering so you don’t need more than the strength of a few fingers but the design remains.
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