Having the RW guys build a CMS solution that ships with RW is a huge waste of their time IMO. And would ultimately be bad for us users.
So many people want so many different things from a CMS system ( fortunately, most can have because of the plethora of CMS plugins out there) that no matter what shipped, you'd want the features of one of the other solutions.
So, this forum would fill up with feature requests along the lines of "XXX CMS plugin from XXX developer can do this, be really cool if the built in RW one did that too, any chance?".
As it is, we all have the best of all worlds: The RW boys focus on what they do, and the CMS devs focus on what they do, doing it in different ways to each other, so us users have tonnes of choice.
I suspect if there was a built-in CMS solution inside RW independent devs would stop work on their own CMS platforms, so ultimately we the user would loose out.