transformers - Why do the evil Decepticons get the best weapons?

I don't disagree with System Down's answer; it's a good in-universe response. But there is also a real-world answer: If the Decepticons didn't have some kind of advantage, there wouldn't be a story.

Before any Transformers movie is made (or, indeed, virtually any Hollywood action movie), there are some guidelines the story is expected to follow: the heroes must be placed in jeopardy, they must appear to be on the verge of defeat, but they must somehow emerge victorious. You can see this structure clearly in sports movies: they are nearly always
about underdogs who somehow achieve an unexpected victory.

So how are you going to set up the next Transformers movie? The Decepticons need to
seem more powerful and dangerous than the Autobots, so they had better start off with
better weaponry. (But it will turn out all right in the end.)