Most of them don't need to. The default project template inserts jCenter() in the buildscript, but most artifacts are proxied from Central.

No I mean that a vast number of the libraries in the Android ecosystem are actually hosted on the free jcenter repository.

Removing jcenter as a source in the buildscript is a trivial step but doesn't remedy the issue as the dependencies are not all mirrored elsewhere.

Complexities of setting up a Maven central account is often listed as the main cause for maintainers simply going for JCenter instead.