They generally don't maintain the software. Things like manual pages and architecture porting can be pushed upstream, and are fine to maintain as diffs if not; buildsystem changes are usually hard, and maintaining that as a delta makes it hard to incorporate new versions.

(One of the packages I maintain in Debian is apparently dead upstream as of the last year or so, but even so I'm not very interested in becoming the new upstream and adopting big changes like a new build system.)