Sure, if someone's not using a popular 3rd party CDN because they don't know the option exists or they don't understand the potential benefits, then yes, consideration should be given. They're not always the answer, though, and there are plenty of legitimate business and security reasons why one might prefer to host their own dependencies.
Sometimes technical reasons make the potential benefit minimal, anyway. Once someone's already making one request for your application source, adding a bit more to that response (a bundled minimum-necessary-subset of some not-massive dependency) can make only a very minor performance difference. It won't be as fast as if a file containing the entire dependency was in the browser's cache (or possibly even in the JS engine's compilation cache), but it'll be significantly faster than if it wasn't.
Sometimes technical reasons make the potential benefit minimal, anyway. Once someone's already making one request for your application source, adding a bit more to that response (a bundled minimum-necessary-subset of some not-massive dependency) can make only a very minor performance difference. It won't be as fast as if a file containing the entire dependency was in the browser's cache (or possibly even in the JS engine's compilation cache), but it'll be significantly faster than if it wasn't.