And they do? Just because they have some flaws doesn't mean that they're completely unusable. If you need an app that isn't on your distro but is on Flatpak, by all means use it.
> practically most packages on Ubuntu or Debian, for example, are outdated.
Some reasonable level of quality control does take time. There are distros that are much more up to date with upstream versions (Arch Linux, Gentoo, etc), but by living on the bleeding edge, you will eventually get cut.
You still need to distribute the static executables and there's no way to update them besides downloading them again unless the software has some auto-updater built-in. That definitely doesn't solve the issue.
I'm not sure I follow your reasoning here. If you are so concerned about the security of your system that you want to run each program on a sandbox, then you definitely do not want to allow programs to "update" themselves by automatically downloading random binaries from the internet.
Well, I never said I was concerned with security. I am to some extent but not to the point where I inspect every single update.
What I value most is having every software or library available through some form of package manager. Downloading static binaries off the internet without auto-update just doesn't make it which is why I like having Flatpak as a fallback.
It also makes it easier for the common user to have every piece of software available through one store. They don't need to know whether it's a .deb or a Flatpak underneath, it just needs to be there and work reasonably well.
There's really no other option: either distro maintainers include every single piece of software in the repositories (unlikely), or we need a common format that works on all distros (already the case with Flatpak).
Snap/Flatpak were meant to solve that issue.
Edit: There's also the fact that practically most packages on Ubuntu or Debian, for example, are outdated.