Transformers - why are they vehicles?!

Transformers - why are they vehicles?

First of all i loved the Transformers cartoon as a child. However it occurred to me recently (im not a child anymore!) why are the transformers in the shape of vehicles from earth? If Transformers are from another planet wouldn't they look like their own native vehicles?

Maybe the answer is in the movie which i havent seen yet (cant wait!)

The answer was at the beginning of the first ever series in the 80's.

The Autobot ship was boarded by the Decepticons in space (many billions of years ago, before humans existed) and during the battle, the controls were damaged during the fight and the ship crashed on Earth (remember the yellow/orange ship stuck into the base of the volcano?) with all of the Autobots and Decepticons deactivated and in bits.
There it lay for millions of years, until an eruption awoke the ship's systems and a Decepticon (can't remember which one now) fell into the range of a beam scanner and repair system. A probe went to explore Earth and scanned all of Earth's vehicles. The repair machine reconstructed the Decepticons (the first one who was rebuilt put his comrades into the beam area) to look like the Earth vehicles and they left to take over the Earth, however Starscream shot at the ship, wanting to leave the Autobots with a farewell gift, accidentally knocking an Autobot into the beam range, letting them get rebuilt with Human machine transformations.