It’s a thing with bazzite and most of the Ublue projects, you can install Flatpaks globally, but they default to per user. It’s designed to be a per-user experience as this is the core of Ublue so you would have say a work image and game image etc rather than the traditional big blob of pakages.
You can get into the weeds and make your own Bazzite! https://github.com/ublue-os/image-template