The security risk their signing process introduces. My guess would be Signal wants a 0% chance of a malicious client being distributed, hence why they only allow direct apk downloads (which self-updates, essentially making an F Droid build obsolete) and Google Play. I would also guess this is why Signal only packages a deb package (if anyone knows a better way to run Signal desktop on fedora [besides the flatpak] than my current solution of spinning up a Mint Virtual Machine [maybe distrobox?] please let me know!) and literally has no official support for rpm based distributions.