Actually, they might. But this is a facet of the Debian build system in
general, and not specific to systemd. You'll find a lot of packages in
Debian where many binary packages are built from a single source
package, and you'll see this same effect for many if not most of them.
It's possible to have separate binary package changelogs within a single
source tree. But it's quite hard, and like other "unusual" things (such
as building packages into the current directory) the Debian build system
rather fights against it.
IMO this level of integration between things which are not mutually
interdependent is a minor bug in itself, but none of the maintainers
are going to agree with me on that.