Because, let's be honest, these days, we're always connected to the internet.
Personally, I've never used the RDoc included on gem installations.
If I need info about a class, method, module, I ask uncle Google.
Besides, it gets kind of annoying, specially when there's gems that take longer to generate the RDoc, than it takes for the actual gem to install.
So, here's a tip for those who don't want to wait for gem to install RDoc you'll never use.
It is as simple as, opening or creating a .gemrc file in your home directory, and adding the following line:
gem: --no-rdoc --no-ri
And that's it. You're setting those options as default when using the gem command. So, the next time you install a gem, it will skip the part where it compiles the gem's documentation.
There you go, enjoy!