Basically, the AUR works by downloading source code and running a build script that builds the app specifically for an Arch system, right? So why isn’t there an equivalent that works on most or all distros? Almost every time, compiling a Linux app is just running the commands that the developer tells you to, and for the uncommon times there are distro-specific differences, they’re usually listed in the README. For many userspace apps, even building across different processor architectures requires little to no change in the commands required as long as there’s a platform specific compiler. Why isn’t there a cross-distro system that can download source code and run a build script while knowing what distro-specific commands to use based on the developer’s instructions? Heck, compiling an app on the system you plan on running it on can unlock a little more performance by taking advantage of compiler optimizations for that specific processor, and it doesn’t take that long on a reasonably modern computer anyway, so why isn’t there an incentive to do it more often?
I think the main problem is dependencies. If a tool is supposed to compile any code for any distro, it needs to support many different package managers. And I would say that Docker or other container software effectively solves this problem (but still needs more work for desktop use).