Once upon a time we built and sold our own CMS, like everyone else, but nowadays the maturity of open source software means it makes most sense for us to build on top of the fantastic open source offerings available instead.
As they're open source they've already had thousands of hours of development effort from some of the brightest minds, and have already been extensively battle tested in the wild in all manner of different setups and scenarios.
You also get to benefit from new features and upgrades created by the community, so even after your project is live there are still people out there adding functionality to the CMS you're using, and should you wish we can upgrade you easily.
It also means the system you're using is well documented, and consequently, there's no vendor lock in, meaning should you decide you don't want to work with us anymore you'd have no trouble finding other developers with experience of the system.
Oh yeah, and of course there's no licensing cost (it's free).
Our CMS builds are based on one of FeinCMS or Django CMS 2 with Django, or N2 or Umbraco with .NET, depending on your requirements. We also have extensive experience of working with the proprietary EPiServer if that's an existing requirement.