Not sure i really understand why this doesn't happen in real planes. Here are two images from MS flightsim that illustrate the phenomenon i'm talking about. The first one shows acentrally mounted HUD ala fighter jet style.
The second shows one slightly offset to the left like in an airliner.
The second one would give the pilot a false sense of where the real horizon is in IMC and would confuse a pilot in clear weather who'd be looking at 2 different horizons.
