I think this question should be a bigger discussion, but these answers are all bogus.
This is also 2 years later of course.
- "it's just one library of many" - include the top 11 then.
- "couldn't agree on common standard" - Kind of making jQuery a standard of it's own at this point.
- "updated more often than browsers" or "make improvements slower" - So the browser won't have jQuery-1.9.x until next browser update, just don't put it in your project yet.
- "Cache anyway" - Sure, it's still a transfer that doesn't have to happen, and there are a lot of people that haven't done a lot of surfing on their new device that you still want your site fast for and so on.
The thing is it is totally doable and would be better for the internet load; by how much is debatable. I could really see chrome at least replacing any net transfer to their CDN with a local copy, but I'm sure there is some legal, security or net neutrality issues with that. Just like I'm sure the main reason has something more to do with such matters and not these lame technical excuses that are obviously not thought through.
This could benefit other libraries too if developers could rely on the speed and availability of a complete library of tools like dojo, and not have to pick and choose just to cut weight. And also as most libraries have adopted the AMD or requireJS approach to package their projects, I believe there is a good argument for the enabling the browser to at least be informed of it's dependencies.