we have a similar problem
it's mostly related to teh age of some of the applcications still in use though. When some were built, everything was seperate, and when the idea of a single customer view finally filtered through, no-one wanted to go back and update them.
So what you end up with is a bastardised hybrid, where instead of linking to the data in the single customer system, any updates to the customer get propagated down to lots of other systems. It's messy and unreliable, but no-one wants to pay to sort it out, it's always going to be fixed in each system "when we replace it", should this mythical event ever occur.