If Android is an opensource OS then why are Android Phones not rooted by default?

Please don't start with statements like, Rooting is not good for an ordinary user, since an ordinary user simply never tries to fiddle with his phone. Rooted/ non rooted doesn't matter to an ordinary user who uses phone only for messaging and phone calls.

The other day I saw a YouTube video which allows me to change the soft button layout and assign custom features to them. But then I saw it works only with Rooted phones. A lot of tricks like blocking ads etc, do work only with rooted phones.

The reason why I ask this question is, because, Its so hard to find a decent 1 click root guide for Nexus 4. And at the xda-developers forums, people talk about downloading toolkit, system images, Custom ROMS and thousand other things, which are simply so hard to follow and seem to be a lengthy process for not so technical users.

When Google allows apps which need superuser permission into Play store why don't they oblige us by giving superuser permission by default?

If Android is truly OPENSOURCE, Why should it be so hard to get "su" working on an Android phone? Its not hard to do superuser stuff on an opensource OS like Ubuntu or Solaris? Doesn't that not negate the whole concept of Opensource when users do not have flexibility?

At least an Android Phone like Nexus 4 which is purely targeted at developers in the first place should be rooted by default is my opinion.