Because I'd probably still prefer to write Javascript than Java. But a GWT approach is one possible solution and there are some Ruby projects that do similar things. None are production ready though. For reference:

http://hotruby.yukoba.jp/ - Implements the YARV (ruby 1.9) bytecode in Javascript (most promising one in my view) http://github.com/mneumann/rubyjs - Compiles a subset of Ruby code to Javascript