Paul Irish points out a huge value added from jQuery that few people (even the jQuery home page) seem to acknowledge, which is that it transparently works around dozens of browser bugs. If you consider older versions of jQuery this is probably more like hundreds of bugs, and many of them have been big showstoppers that even crashed browsers.
Fans of the “vanilla.js” movement—an understandable reaction to mindless library use—tend to understate what a minefield browser APIs tend to be when you consider the wide variety of browsers in use (take a sobering dip into your site’s browser stats). There’s nothing wrong with using and learning the real APIs, of course. There are big sections of DOM, Events, and friends that are very well supported, but if you’re not extremely careful, as soon as your site picks up traffic you can count on “vanilla” code breaking for users with browsers that don’t auto-update.
The moral: Even if you’re a JS pro and know the native APIs, if you’re doing anything substantial with them, then using jQuery—or some other battle-tested library—isn’t so mindless; you’re improving UX for some end users.
Why doesn’t someone create a library that just fixes the bugs? Dean Edwards was an early experimenter here with base2 and IE7.js, but as far as I remember these were mostly combed over by the JS nerds who were working on their own libs. Combining workarounds with other useful bits just gave you more bang for the buck, and, frankly, the standard APIs like DOM have never been particularly elegant to work with anyway (it took the W3C way too long to even acknowledge that giving devs the ability to parse markup (innerHTML) was something browsers might want to offer!).