I can point one specific example with libre office: 3.9GB for the pack vs 785MB for the .deb.
You already have most of the major dependencies installed natively as they are depended on for many other packages, and you're not including the space they take up as part of installing the native package, but you are including them as part of the flatpak.
When I first started using it, one of the talking points was that Linux kept the system clean of clutter and that improved longevity for the hardware and delivered stability by not having unnecessary and unused or orphaned and redundant libraries and dependencies.
Flatpaks literally improve this. The core system itself remains extremely minimal and lean when you use containers, in both the server and desktop space. This greatly improves stability and longevity. We all know how much of a pain it is to do a point release upgrade on a system with tons of installed software. Flatpaks do not have this problem because they are independent of the system and each other.
but we are carting in a ton of junk that should not be necessary
It is necessary, and it's not junk.
Debian wiki links to this to further educate/alert on the down sides of flatpacks.
Much like Debian packages, the Debian wiki is stale and outdated.