May be 'sample' is not a good name.

Of course you should be able to check out the project, do a `clean build` and everything should work, actually it does in the current setup.
But after that may be you want to do some little tweaks in the source (not the sample source) and see if it still works. You can't do that if the sample does not use the source next to it. So there is a barrier to collaboration. 
And actually it is not completely trivial to link to the source instead of some deployed version. You need a `buildSrc` directory and some specific gradle setup (see `build.gradle` and `settings.gradle`). Or is there an easy way to set that up?