Everywhere, in the 1980 animation, Animated, Prime, RID2015 and comics they have their Cybertronian form and it is most often a vehicle. But, why? Because they don't have any reason to convert themselves to any mode of transport unless they're lugging around something, and then they would be among the likes of OP with attachable parts. Why have the cockpit at all for Starscream and other jets? For the Quintessons? Wouldn't they look different in that case, with the different anatomy? And they don't even exist in every universe, Prime and Animated for example. I can see the transformation being only because alt modes provide faster travel from place A to place B but that doesn't require seats for nonexistent people. And the Robots in Disguise charade i can see after they arrive to earth.