Because that isn't their purpose

When you have debian folks removing .packlist files and .bs files ... more files and fatter installs just aren't welcome

You can always allow/cofigure/direct your cpan to keep your build directory really really big and not delete any build files, so you'll have all the README/INSTALL/META....

Then there is always metacpan API to find out meta stuff ... see Re: RFC: QA Uploads ( yes, cpanmetadb cpanminus cpanm --latest-unauthorized )

There is also http://deps.cpantesters.org/ which is open sources ... this means you can run your own cpan mirror (or mini mirror) or only mirror of approved modules ... and can create dependency graphs and trees

So there are solutions out there if you want them :)