Distributors entering Flatpakland

> Every single one of the security benefits claimed by flatpack could be provided by traditional packaging systems like dpkg and rpm as an incremental improvement to those packages.

Sure, but are they doing it? Almost universally no, because this kind of sandboxing for linux GUI applications requires a lot of additional work, metada, code changes and considerations and distros don't see a need. As far as I can tell, Flatpak have been the only ones driving these features. It does happen a bit for system services though thanks to systemd's sandboxing features. But getting every maintainer to turn them on has been hard.

If you want to change that go ahead, it would be great!

> So why flatpack in the first place, if it comes with all this enormous baggage?

The primary goal is to make it easier to target applications at Linux. The biggest demographic of Flatpak developers, afaict, is KDE and GNOME developers who were annoyed by the amount of work required to get your application onto every distro and inability to deliver speedy updates. Isolation and sandboxing are just implementation details to make that less haphazard.