[dozzie]

> [...] most of the time [installation instruction] boils down to adding a ppa / key, running curl to get some other script / binary etc

> Why can't we apt-get / brew install everything yet?

And you think this awful PPA of yours is what exactly?

> Or why doesn't websites just give a `install.sh` file with all the relevant instructions in it?

Try writing such install.sh. But remember that it has to work on Red Hat, Fedora, Debian, Ubuntu, Arch, Gentoo, FreeBSD, NetBSD, MacOS, and Cygwin, each with different package manager, paths, and major versions of your dependencies.

> This quickly gets repetitive especially when setting up several computers.

Which is why a skilled sysadmin provides his own repositories along with the ones from his distribution.