Well, that's the thing -- there are good CDNs out there, but they expect you to pay. But why is the developer making the decision of making the user pay for it with their privacy? Without any kind of notification or agreement, or even discussion, really, with the people who should be deciding this stuff.
Now perhaps I'm wrong, and this is not a developer decision at all but a decision from the business side, where they weighed the privacy issues against the slower performance against the cost of paying for a CDN, and decided this was the right way to go, then they instructed their developers to start loading all these scripts from third party free CDNs.
Except, I've been doing this thing for over a decade, and developers keep sneaking this shit in, and then acting surprised that it's not OK to pull jQuery from ye-favorite-free-CDN, but they need to stick the thing into static resources where it will be served from our CDN. And I keep finding devs doing this also when I do pentests of third party sites, and the response from the security POC is usually "we had no idea this was being loaded from the free CDN. We even have a contract to use Akamai, we just didn't know".
At the same time, I keep seeing this in open source software, even in examples and tutorials, where supposedly the user speed is not so crucial, as well as stackoverflow, and I also know there is a lot of cut and paste going on, so I still think this is just bad hygiene on the part of the developer community, where people are just very cavalier about letting third parties inject javascript into your origin.
But you do have a fair point. End of rant.