Convenience cost. The parts are small. They put it together, find the right sized container, figure out what can fit together nicely, so you don't have to spend the time customizing a kit yourself.
Of course, this also likely means you don't know how to use the items in the kit all that well, and the tools will not be as good as what you could get and put together for yourself for the same price.
This is not "you're an experienced prepper that wants the best, buy this", since that market would just make their own. This is for those starting out, or for those with very little time.