The upcoming movie, fourth in the Terminator series, is expected to be a sort of prequel, despite being set entirely in the future. Chronicling the evolution and rise of the Terminator robots, the new film details the events that led up to the need for time travel in the previous movies.
The vehicular form of the spawn of the infamously sentient Skynet defense system are still early in their development cycle, and have not yet arrived at a need for a true humanoid form. Instead they embody the form of the machine that's best suited to their primary task, and in the case of the vehicle-based moto, truck and air terminators, that's usually killing with efficiency and speed.
Source: Motor Authority