I agree with mike.fro. Almost every site has something that makes it's needs unique. Yes, you could write a module for some of the other CMS's, but, by the time you figured out their complex module scheme, it might just have been easier to write a custom CMS.
I find that most cms's aren't flexible enough to do what my clients want to do. The fact that they always want things tweaked, or major alterations in functionality. It's just alot easier to go with custom.

Brett