Category Five is in a...uh... category all its own.
First of all, the companies that have historically compiled these collections are either out of business, charging a yearly license fee or offering their library on a barter-only basis. In short, very few libraries can be had on a cash buyout basis.
In addition, many libraries are sold as "service packages", lumped in with other production elements that may or may not be useful. If you're in a small market, this might not be a bad thing because you're saving time and money on resources you can't afford in-house. But if you're in a medium or large market, you're paying tens of thousands of dollars a year in spot revenue simply to get a few sounds. Oh yeah, and if you cancel your contract, say goodbye to the sounds.
The only other options are those $50-$100 sound effects or sample libraries that may offer a few decent sounds, but at such a low price, you're getting very little content.
In other words, conditions were perfect to spin off Category Five--a new force on the sonic landscape that's broad-sounding, totally superior in quality and available on a buyout basis.