One problem with a motorcyclist riding directly next to another vehicle is that:
The other vehicle may block the motorcyclist's escape route.
The motorcyclist can be easily seen by other drivers.
The motorcyclist may not be able to read signs on the side of the road.
Riding alongside another vehicle is dangerous because the vehicle could veer into your lane and sideswipe you. Additionally, the vehicle could block your escape route if a hazard arises.