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.
This makes a leetle more sense if you watch the very first episode. I'm going to do my best to give an answer without writing too much of a lecture:
In the first episode we see bumblebee and another autobot transform into their original transformations- which were NOT earthly vehicles. After their space ship crash lands on earth, all of the transformers were deactivated. However, in the 80s the Arc was reactivated. The first thing that it did was to enable camouflage protocols and alter the bodies of all of the transformers present (both autobots and decepticons) into similarly functioned earthly equivalents.
Remember that the autobots and decepticons were slaves- those symbols? Slave brands. The autobots were the industry slaves, while the decepticons were military slaves. This is why one group has more battle-style transformations (jet fighters) and the other more transportation-style transformations (a truck, a microscope, etc)
Edit: since I'm being doubted as to the second point, I will quote from the transformers wiki (which includes episode references):
In ages long past, a race of aliens called the Quintessons existed on the planet-sized factory known as Cybertron. [Five Faces of Darkness, Part 4] Some of their early experiments in creating artificial life resulted in the Trans-Organics. [The Dweller in the Depths] Some Quintessons turned to forbidden arts, and were banished to other dimensions. Madman's Paradise The Quintessons eventually created two lines of sentient robots: one was a line of robots designed for domestic duties and labor (known as Autobots), and the other was a line designed for military use (known as Decepticons). [Five Faces of Darkness, Part 4]
Hot Rod at one point also says
We Transformers have looked into the face of our creators... and seen the face of an enemy. Rodimus Prime, "Five Faces of Darkness, Part 5"
Bear in mind, we see in an episode which gives the back story to Omega Supreme that Decepticons did not used to be evil (which, oddly, in the second Bayformers movie was alluded to with the offhand "it's a lifestyle choice" line). Megatron would "infect" his fellow transformers to turn them evil. There was a process to temporarily revert them (as was demonstrated in the episode with the Constructicons) but once Megatron had his hands on the mind of a Transformer, they could never be changed back permanently.
And so it makes sense that Megatron would go for the Decepticons first, since they were the military branch. The Autobots were barely keeping the resistance alive because they had better tacticians and such, but the Decepticons were very much winning the war just through sheer power.
The entire reason of them turning into vehicles is so that they will be in a cameo suitable for that planet they are on but here's how that process works:
They are not able to choose a vehicle that is either to large are to small to fit into their previous mold, or at least have the same mass... This is why the vehicles they choose look similar are at least the same size as their cybertronian vehicles.
The transformers will most likely pick the one that looks most like their previous for so that they can adapt easiest to it... They then scan the vehicle and mimic it... Not exact though.
The reason I said not exact is because even though they may look identical the nano structure of everything is different from the actual... Therefore it's a copy but not the real thing... Kind of like a picture you can physically feel... It looks identical but isn't the actual thing
On cybertron vehicle forms were used to haul, transport, and carry weapons and such... Most of the time unless it was meant to be a hauler they had no inside opening that was humanoid size... It just so happens that earth vehicles look close to the design of the cybertronian vehicles... On earth the transformers have to have that space so that they will be in true camouflage with the world around them.
Finally, I deny what he said about the slave thing... Before the war there were no symbols on their arm or chest because they did not need them... Optimus states "Two factions rose..." One for peace and one for war... That's why they have emblems... That's also why the autobots have a peaceful symbol of truth and wisdom, and the deceptions have a symbol of hatred and war.