Thanks in large part to the ... alternate mode and the real truck. Incidentally, there's a somewhat popular secondary theory ...

Lego's 1,508-piece Optimus Prime model can transform into a truck, which makes it one of the coolest Lego builds around.

Though this might fit better with the Beast Wars Maximal Optimus Primal, the toy splits open through a lever to transform into Optimus Prime in robot mode, complete with a "truck" torso.