Because that's traditionally how open source has been done? For decades? Especially if you're in a Unix/Linux environment? Eventually we started getting package

Because that's traditionally how open source has been done? For decades? Especially if you're in a Unix/Linux environment? Eventually we started getting package managers and the distro maintainers started creating binaries of most of the packages you'd want to install, but the distro maintainers usually build those packages from source. This is new software and I imagine the distro maintainers will package it up if it starts to gain steam. Doing it from source also has the advantage that the package maintainers can customize the build process so that it works better with their system. Photoshoto and MS Word are closed source and proprietary, which creates issues if you want to package them.