Why gem 'therubyracer' is commented by default in Gemfile when we require to uncomment to start rails server?

Why gem 'therubyracer' is commented by default in Gemfile when we require
to uncomment to start rails server?

I created one rails new application and tried to start the rails server. I
got following error:
/home/ritesh/.rvm/gems/ruby-1.9.3-p429/gems/execjs-2.0.1/lib/execjs/runtimes.rb:51:in
`autodetect': Could not find a JavaScript runtime. See
https://github.com/sstephenson/execjs for a list of available runtimes.
(ExecJS::RuntimeUnavailable)
from
/home/ritesh/.rvm/gems/ruby-1.9.3-p429/gems/execjs-2.0.1/lib/execjs.rb:5:in
`<module:ExecJS>'
from
/home/ritesh/.rvm/gems/ruby-1.9.3-p429/gems/execjs-2.0.1/lib/execjs.rb:4:in
`<top (required)>'
from
/home/ritesh/.rvm/gems/ruby-1.9.3-p429/gems/uglifier-2.2.1/lib/uglifier.rb:3:in
`require'
from
/home/ritesh/.rvm/gems/ruby-1.9.3-p429/gems/uglifier-2.2.1/lib/uglifier.rb:3:in
`<top (required)>'
from
/home/ritesh/.rvm/gems/ruby-1.9.3-p429@global/gems/bundler-1.3.5/lib/bundler/runtime.rb:72:in
`require'
from
/home/ritesh/.rvm/gems/ruby-1.9.3-p429@global/gems/bundler-1.3.5/lib/bundler/runtime.rb:72:in
`block (2 levels) in require'
from
/home/ritesh/.rvm/gems/ruby-1.9.3-p429@global/gems/bundler-1.3.5/lib/bundler/runtime.rb:70:in
`each'
from
/home/ritesh/.rvm/gems/ruby-1.9.3-p429@global/gems/bundler-1.3.5/lib/bundler/runtime.rb:70:in
`block in require'
from
/home/ritesh/.rvm/gems/ruby-1.9.3-p429@global/gems/bundler-1.3.5/lib/bundler/runtime.rb:59:in
`each'
from
/home/ritesh/.rvm/gems/ruby-1.9.3-p429@global/gems/bundler-1.3.5/lib/bundler/runtime.rb:59:in
`require'
from
/home/ritesh/.rvm/gems/ruby-1.9.3-p429@global/gems/bundler-1.3.5/lib/bundler.rb:132:in
`require'
from /home/ritesh/projects/interview/config/application.rb:7:in `<top
(required)>'
from
/home/ritesh/.rvm/gems/ruby-1.9.3-p429/gems/railties-4.0.0/lib/rails/commands.rb:76:in
`require'
from
/home/ritesh/.rvm/gems/ruby-1.9.3-p429/gems/railties-4.0.0/lib/rails/commands.rb:76:in
`block in <top (required)>'
from
/home/ritesh/.rvm/gems/ruby-1.9.3-p429/gems/railties-4.0.0/lib/rails/commands.rb:73:in
`tap'
from
/home/ritesh/.rvm/gems/ruby-1.9.3-p429/gems/railties-4.0.0/lib/rails/commands.rb:73:in
`<top (required)>'
from bin/rails:4:in `require'
from bin/rails:4:in `<main>'
Although I was able to fix it by removing the comment from line 'gem
'therubyracer', platforms: :ruby' from Gemfile.
Why this gem is commented when we require it at very first place when
starting rails application?