I like the idea behind Flatpak and the ability to sandbox applications. What I don't understand is why they chose to lock the build process so tightly to Linux.

I'm currently on macOS, and while I can cross-compile applications, I can't actually bundle them as Flatpaks. For that, I need `flatpak-builder`, which is so deeply coupled to Linux itself that I don't think it can realistically run on other platforms.

Snaps have a similar issue with `snapcraft`. However, you can still build a snap manually with `mksquashfs`, whereas I haven't found an equivalent low-level option for Flatpak. I may be missing something, though, and I'm still looking.