For every dependency that's missing from the upstream distribution in the exact version we need, we'd need to package that appropriately. We have nothing to gain here, nobody pays for that.
Once you package a dependency, updating to a new version requires minor changes unless it's a major version change. What you do gain is the ability to easily upgrade and downgrade a particular dependency and verify the integrity of the installed files (something that pip, for example, doesn't provide as far as I'm aware, but the OS package manager does).
This is just a crazy amount of work, given the alternative is updating your whatever lock file and let the transitive dependency resolution of your language of choice do the rest. Note that I very much like proper clean .debs as an end user, and if that's my customer base I'd publish like that as well. But if I'm last in the chain and my customers are intern, I'd never ever in a million years take the route you propose.
