The story of the Skrulls was not exactly planned out by Marvel. It's entirely possible that Stan Lee and Jack Kirby never intended to use them again after their first appearance back in FF #2, but they stuck around and have popped up all over the Marvel universe in succeeding decades.
But I don't think anyone has ever really put all the pieces together and considered how Skrull culture actually works. Now I don't pretend to know all the details; I certainly haven't read every Skrull related story, and it's been years since I last read some, so set me straight if I'm missing something.
When the Skrulls first appeared in FF#2 (I'm discounting their retconned previous appearance in Marvel: The Lost Generation as the whole series seems to have been written out of continuity now) they pretend to be the Fantastic Four. They can shape-change, but do so only to copy the FF. In fact it is apparent that they have total control over their body shape, but they only use this to stretch when pretending to be Reed Richards.
Ultimately, and I'm sorry if I'm spoiling this for anyone, they are defeated by transparent lies and fooled into thinking that comic book art is a photographic representation of reality. CGI would be wasted on the Skrulls - they are completely taken in by line art and a four colour dot screen.
Looking at this now it seems absurd that they could possibly be fooled. The explanation given is that their eyesight is poor. I don't believe it. There is no way they could possibly have duplicated the physical forms of the FF so accurately if they are myopic enough to be unable to distinguish between comic art and photos. So what's the real reason?
Quite simply, the Skrulls have no concept of fiction. They are entirely devoid of creativity. If there is a picture of something it must be real because they cannot comprehend the concept of making a picture of something that does not exist. This is their one blind spot and the thing that cripples them. They have the ability to be anything they can think of, but they do not have the imagination to do anything other than copy the things around them. Sure, they have an empire. But I suspect if you looked into Skrull history you'd find they were just copying the imperialist activities of the Kree.
Look at how Skrulls are presented when we get occasional glimpses into their society. In our world if everyone could shapechange, it would all be about fashion - everyone would look like the latest celebrity, fads in physical shapes would sweep through the world. Society would fall apart because nobody would be able to keep track of who everyone else was as they changed shape at whim. But the Skrulls keep their own shapes unless they have a reason to change. And they use tools when they could reshape their own bodies to be tools. In fact it is their lack of imagination that allows their society to function.