I haven’t seen the movie itself, so this is based on the related comics - 4 issue prequel, 4 issue adaptation, 1 issue Target-exclusive prequel.
[QUOTE=Diogenes the Cynic]
Who made these robots and why?
[/quote]
It changes from continuity to continuity.
In the movie continuity (as presented in the comics, anyway), they were gifted with life by the All-Spark. Whether this was an accident, or a deliberate choice by the All-Spark is not clear. The Cybertronians don’t know, but it doesn’t really matter to them - the All-Spark is the source of their life. That’s all that matters.
Camouflage. They don’t want to be noticed and interfered with by the natives, and avoiding the other faction’s notice is also a good thing.
Yes, as long as the mass is about right - they see something that strikes their fancy, and choose it. Bumblebee even changes the model of Camaro he is at one point.
Because their camo forms are chosen based upon the 'bot’s personality, their mass, their primary function (Frenzy, the 'Con’s espionage specialist, disguises himself as a CD player and PDA at various points), and just random chance in what they happen to run into when they’re chosing it.
Barricade’s as heavily armed as Starscream or Blackout, despite his choosing to disguise himself as a police car rather than a military vehicle, and it gets him better access to more locations - a police cruiser’s going to be less conspicuous in the middle of a city than an F-16.
'Cons tend toward more militaristic forms, simply because they’re more militaristic.
Megatron wanted the power of the All-Spark, and recruited the other 'Cons to help him. Since this would amount to absolute power over all Cybertronians, the 'Bots obviously didn’t want him to have it.
Other continuities are similar - generally, the Decepticons start out trying for power, and later on, after being defeated, they, or their descendants, the Predicons, go for revenge on the Autobots/Maximals.
Not an issue in the movie, as the camo forms and bot designs were chosen specifically to avoid the issue. (Frenzy becomes a PDA after his head is separated from his body.) The more recent TV series and toy lines have also tried to keep scale in mind. Ignored in the original cartoon, handwaved away in the IDW mainline comics.