Cause .tar.gz is not an executable binary in itself. It is more like a zip of the binary and its support files. Since it would be really difficult to distribute and maintain native binaries for every type of distro, application publishers used to just distribute for distros that has a larger userbase, like a .deb for Debian/Ubuntu and a .rpm for Fedora/RHEL/Suse, and then they would provide a tarball for any other distro. Flatpak and Snap have been introduced to eliminate this overhead from application developers, but they still offer tarballs as the newer solutions are still far from standardization.