I guess the are inside some artefacts which are not part of the actual sdkresources. You need to download them with the SDK-Manager.
BUT it is not recommended. As soon as you update all needed artefacts you´ll break something in the compilation/running your app.
I tried it recently but ran into problems so i directly stopped and reinstalled the SDK (not starting the sdkmanager new).
Edit: If you start the sdk-manager and just search for MLKIT you should find some artefacts. Be aware to reinstall the SDK on problems.