I actually think the responders on here fail to understand that the answer to the question is browsers probably should look to include the most widely used libraries, and polyfills etc on the client side.
As the person asking the question states, conditional comments could be used to ensure that those using browsers that don't include jquery were served an appropriate version.
Jquery also includes it's own backwards compatibility support through migrate, allowing for conditional comments to give retro support for someone with a packaged version of an older jquery library without downloading a whole new library.
The argument FOR including these in browsers would be not just about user experience and cost, but also about this planet we live on. The use of data is a huge contribution to global pollution, and ensuring that needless data transfer is minimised could have a dramatic effect on our carbon footprint.
In essence for the sake of adding an additional few megabytes of code bloat in a packaged browser - the same data is being needlessly ferried about billions of times a day.
That worsens user experience for everyone on the internet. And costs major businesses huge amounts of money.
As a developer you would just create the necessary fallbacks required as we do currently for IE etc, so what's the problem - it probably should be included surely?