Question: If XML Repositories are so great, then why doesn't anyone market an XML repository as an LCMS?
Answer: Practically all LCMS vendors are organized according to a service consulting business model. They invest massive amounts of time and money to create efficient systems, which they practically give away for free. They do this so that they can sell you customizations, service, training, maintenance, and support. A pure XML repository system could be serviced and maintained by a wide variety of vendors. They might never earn back the investment they made in creating the solution.
The proprietary product offer does tie the business to the vendor, but it also ties the vendor to the business. The vendor has a huge stake in the outcome of the LCMS implementation.
There are some pure XML repository LCMS solutions that have been developed by the Open Source community (principally by and for academic institutions). They are more like do-it-yourself kits than a fully-developed product offering and do not offer the reliability, features, or performance of COTS4 solutions.