I think, you’ve answered your own question? There’s a lot of different formats for Linux. Getting them all correct and working on the different distributions is significantly trickier than just bundling a self-contained archive.

Having said that, they do actually provide a DEB repo since a few months ago: https://support.mozilla.org/en-US/kb/install-firefox-linux#w_install-firefox-deb-package-for-debian-based-distributions-recommended