The MIT license requires attribution (in the form of the copyright notice and the license file) for the MIT licensed code.

But it doesn't require that other code in the project be released under the MIT license (or even released at all), which is what copyleft would do.

It's pretty close to releasing it to the public domain, and the point of that is that it's actually really hard to release something to the public domain worldwide. Just saying something like "I release this code to the public domain" doesn't have the same effect in every country.