My counter examples:
- Small cars, like the SMART, are not much longer. Some sports cars like the Morgan and Lotus Super 7 are also not much longer. Should they be allowed with one person?
- Throughput is probably the real reason. The designers would have figured traffic flow based on the space needed to safely move two people. Since at 100km/h this is around 30m/s then any vehicle is going to use about 60m of roadway. Unless you're driving two bikes side by side, you are a HOV Hog.