

If a distribution didn’t come with any default software, it would be unusable out of the box by the average user. If you want that, choose something like Arch or Gentoo or LFS.
The point of FOSS is freedom and choice. You can choose a distribution that aligns with your needs, and once it’s running, you can e.g. replace Firefox with Chrome or use Nautilus instead of Thunar. Try uninstalling IE/Edge or Explorer on Windows for a direct comparison.

Flatpak is great for two groups of users: the ones who only use default settings in standalone apps and the privacy-oriented experts who know how to tweak things to their liking. In the middle is a large group of users who don’t know or care how things work, but they want that one feature an app is supposed to do but mysteriously doesn’t work with flatpak.
Even one of these occurrences is enough to make most users give up on that app or the OS entirely. I like the idea of sandboxing apps, and I use flatpak daily, but we have to acknowledge and hopefully improve some of its limitations or many users (yourself included, it seems) will consider it unusable.