Like, most of the way deployment should work is that you come up with some collection of packages that need to be installed and you iterate through and install them. Bob's your uncle.
Thing is, all the packages you need live out in the wild internet. Ideally, you'd just be able to take a package, vet it, and put it in your local artifact store and then when your production deployment system (using apt or yum or pip or gems or maven or whatever) needs a package, it looks at your local artifact store and grabs it and goes about its business. Never knowing or touching the outside world.
And your developers would all write their apps to deploy through the normal packaging methods that everyone and their mother is already familiar with and they could just put them into the existing package index as well.
But you've gotta lay out pretty serious moola (from when I last looked into available solutions to this) or set up a half dozen different artifact stores if you want to do things that way. And good luck managing your cached and private artifacts if you do. And on top of that developers don't necessarily know how to set up a PyPi or a RPM index or whatever so that the storage is reliable and you've got the right security settings or whatever else. (I know I sure don't and I'm not really interested in reading all of the ones I'd end up needing).