Dependencies across different language ecosystems exist (for eg Python stuff often depends on JavaScript stuff for documentation, or C libraries for faster machine code), it is convenient to encode them all in one package manager format. Repackaging ecosystem libraries is pretty much automated these days but still exposes you to the internals of each library since you need to do QA on everything to get it up to Debian standards first. Once things are in Debian you get an entire community of QA too, checking for new build failures due to changes in other packages, notifying you of new security issues etc.

https://wiki.debian.org/AutomaticPackagingTools