What Marker said
Plus, what Boeing did to build the LCFs was buy back some well used 747-400s (passenger models) and sent them to Taiwan (I think, not absolutely positive) where the actual conversion was done. The work was done to Boeing drawings and engineering, but aside from a few managers and inspectors, the conversion work was done by the contractors.
In addition, the design itself was less than elegant - for example that big pressure bulkhead in front of the cargo section is basically just a big flat plate of aluminum - not the usual dome shape used for pressure bulkheads.
Also, the LCF was certified to a special purpose STC which is less rigorous than a full blown Amended Type Cert that would have been required for a production airplane.
Originally they were going to build 3 LCFs, but that was for a 7/month production rate - when it was decided to increased the production rate to 10/month based on the popularity of the 787, it was decided to build a fourth LCF. There was some serious lobbying going on a few years ago to build a fifth while we had the chance, but they couldn't get the bean counters to go along with it.
I'm sure there are some Boeing managers who's worse nightmare is that an LCF is seriously damaged (or worse) and out of service for an extended period - that would really throw the proverbial monkey wrench in the 787 production.