system
Given the following post, respond as if you're commenting on Reddit:
human
[Transformers] The transformers are from outer space, right? So why do they turn into regular Earth cars?

Why not some sort of alien cars?  


And if they chose the disguises based on what was common here, why do they stick to a single disguise each?
gpt
In the first episode of the original cartoon the initial scenes are on Cybertron and they are transforming into alien cars and planes (persumably Cybertronian cars and planes) . When on Earth they convert to human cars and planes to disguise themselves.

Hence, Robots in disguise.