Reviewing upstream packages to write debian/copyright files is tedious manual work. It tends not to get done again after initial packaging, especially not on every release (when something may have changed).

Making initial copyright file construction, and subsequent review/update easier will improve Debian's software quality.

Stretch (Debian 9) has significantly improved tools over previous releases.

scan-copyrights can update an existing copyright file from rescanning the source. It can also create one from scratch. It uses the underlying licensecheck tool from devscripts

The package needed is cme (and libconfig-model-dpkg-perl v2.074 or later)

updating is as simple as: cme update dpkg-copyright

More details here: https://github.com/dod38fr/config-model/wiki/Updating-debian-copyright-file-with-cme