Ask HN: Why isn't jQuery included in the browser?

There is no such thing as "the browser" - there are lots of browsers, each of which has many versions in current use (not everyone can auto-update to today's version of Chrome, especially in big companies). Since you have no control over which browser your users are running, what do you do if your web site is dependent on the latest version of jQuery but half your users have a browser that's several months older than that?

On the other hand, if your web pages load a known version of jQuery, you have control over the dependencies and can update to a new version of jQuery when it's convenient for you to do so (and after you've tested it to make sure it doesn't break your site).

And jQuery is just one of dozens of popular JavaScript frameworks...

In practice, it kind of is. Because of the caching. This mechanism also adapts to possible future changes. Today jQuery is popular, tomorrow it might be something else.