Seems like an administrative nightmare. Incorrectly loading JS libraries is just one of many problems when your network connections is compromised.
I actually even always wondered why the default css applied to elements isn't standardised, so pages not containing reset.css or normalize.css-sheets render differently or how certain Javascript methods differ from browser to browser.
But I guess that is a rather different discussion.
So, different browsers would have different libraries included depending on who made them. Possibly different versions too. It would just be very messy, with little reward.