Because Google download dosen't provide source default. Developer needs some configuration to access source code.
We can just download the latest source code by just entering the URL in the browser or clicking on the following link.
http://git.source.android.com/?p=platform/frameworks/base.git;a=snapshot;h=HEAD;sf=tgz
1. create a fold "sources" at the same directory of android.jar
2. copy the relevant source code to the new created fold
Then reopen your eclipse. You probably can accesss source code now.