I always wondered if Debian/Ubuntu could benefit from a "monorepo". It seems to work for other distributions, e.g. Alpine Linux and Homebrew.
https://github.com/alpinelinux/aports/tree/master/main
Right now every Debian package lives in a separate repo, or it doesn't even have to live in a repo at all AFAIK.
I think Debian has the most packages because their process is very loose and decoupled (as well as it being one of the oldest distros). But having tighter integration does help move things forward faster.