Instead of implementing the whole of jQuery core, they could just add some of the features. It seems like that's already being done though (e.g. Element.querySelector()).
Think about the places where the average application uses compression codecs.
Now, think about the kinds of applications people are trying to build in web browsers.
If you were to represent these two groups as a venn diagram, you would have one giant honkin' circle.
(A few examples for the lazy: On-the-fly content compression/decompression for games; caching large amounts of application data in compressed form in order to reduce disk space usage; using compression codecs that are faster or produce better compression ratios than gzip.)