They are not separated
like for example Debian does it with their -dev version packages.
For the exact reasons someone else has to explain. I guess it's
something about grafting, reproducibility, source provision etc.

Sometimes it is more appropriate to separate the various types of files
produced from a single source package into separate outputs.  For
instance, the GLib C library (used by GTK+ and related packages)
installs more than 20 MiB of reference documentation as HTML pages.
To save space for users who do not need it, the documentation goes to a
separate output, called @code{doc}.  [...]

Some packages install programs with different ``dependency footprints''.
[…] This allows users
who do not need the GUIs to save space.  […]

-- 
Regards
Hartmut Goebel

| Hartmut Goebel          | h.goebel@crazy-compilers.com               |
| www.crazy-compilers.com | compilers which you thought are impossible |