Because there is no single build that will work on all 'Nix OS’s, and we don’t have the resources to build and test multiple versions for Linux. Currently there isn’t even agreement about “universal” packages, as Flatpak, Snapd, AppImage, PortableLinuxApps, Zero Install, Autopackage … vie for dominance.
For users of Ubuntu 20.04, and derivatives, I’ve posted simple step-by-step instructions here: Building Audacity 2.4.2 on Ubuntu 20.04
Are you volunteering?