Because not everyone has a Ruby development environment available. It would be a lot better idea to stick with Makefiles or other more commonly available Unix tools.
A lot of Ruby people seem to add a Rakefile or a Ruby script to do some simple task like installing a few dotfiles. That makes them a lot less useful to me (for example), as I don't have a Ruby env available on all machines I use.
Same goes for Node.js, Python and Perl people. If a "standard unix" tool can do something almost as easily as a Rakefile/whatever, you should probably stick to the standard tools. I don't know what you should do with Windows, though.