I would agree with you if there wasn't already RPM, debs, etc. But also given how flatpak is implemented on top of containers, it's basically "sandboxed by default" and any unsandboxing involves exposing/mounting things in from the host into the container. You can "unsandbox" any flatpak you want using flatseal (or if you know how, flatpak directly), so I think flatpak is actually pretty good in this regard.