The universal packaging problem is having all the dependencies for all the software, making it all available for end users, in one place and easily make those packages for virtually every distro by developers wanting to target Linux systems. A lot of companies do not support Linux for this reason: there is no one single format to distribute software (not considering tar.gz).
Having a tool that convert packages to other distro’s package format does not solve this, it’s just a long chain which original devs of a software are not in control of. The idea of Flathub and Snapcraft (the repos, not flatpak/snap) is to make one place for third party devs to publish their software (foss and not).
Also, flatpak is trying to make a “standard” api to interact with the host system thru the sandbox for privacy/security reasons (like android).