Developers are lazy, thus Flatpak
So the thing Flatpak is supposed to fix for me as developer is that I don't need to care about distributions anymore. I can bolt on whatever dependencies I want to my app and it's dealt with. I also don't need to worry about having software in distributions, if it's in Flatpak it's everywhere. Flatpak gives me that unified base to work on and everything will be perfect. World hunger will be solved. Finally piece on earth.
Sadly there's reality. The reality is to get away from the evil distributions the Flatpak creators have made... another distribution. It is not a particularly good distribution, it doesn't have a decent package manager. It doesn't have a system that makes it easy to do packaging. The developer interface is painfully shoehorned into Github workflows and it adds all the downsides of containerisation.