[eluser]BrianDHall[/eluser]
The short answer - just about everyone likes ice cream, but most of us prefer different flavors. I like the kind of chocolate flavor that makes other people sick, and most people are OK with vanilla but few people really love it. Similarly, with user auth there are many approaches and systems and some applications don't need it, some need only a basic auth system, and some benefit from a full fledged authorization system worthy of the most complex CMS software.
CI is not a very opinionated framework in general, and EllisLabs seems to have figured that user auth was just too opinionated for their tastes to provide a bundled system.
Over in IgnitedCode there are lots of auth libraries available. Personally, I just make my own as it is about as simple and straightforward as you can get and I have very specific ideas in mind for how I want my auth to behave.
