Actually it did happen with RPM, at the developer level integrating changes and cooperating. Features like delta-rpm's eventually got included in Fedora.
Probably this happens because developing your own, gets more kudos, is more interesting and is much more sexy.. than integrating in someone elses code and then integrating changes back upstream.
As the package management, has been a key value added feature of distro's, bringing in a tool from outside, likely has high bar to clear. After all each resolver & update system has grown organically in it's own niche distro environment and been shaped by it.
Let's just hope, it doesn't get added into 'systemd' (*joke*)