Bbig cats are everywhere. I cannot think of a reservoir where they aren't . Chatfield for example, Cherry Creek. When people say there aren't any there, they haven't exploited the entire environment. There are catfish so big they break lines with regularity.
The difficulty is that fishermen try a ONE-SIZE-FITS-ALL mentality. That is, they use gear and bait for little eating size cats and wonder why they don't catch the monsters with that stuff. Then, if they fish for monsters without the proper bait and gear, or choose a poor location, they think there are no monsters there.
You can apply the same logic to about any fish species. You can catch small trout of 12" and under all day; but that isn't using the same technique that will land a trout of 5 pounds and up, necessarily. I believe the adage applies, that you get what you AIM for.