Because when you're buying a "major car OEMs are happy to sell this number" number of vehicles you can afford to get exactly what you want and the resultant few % reduction in operating cost over the following decade or two will more than make up for the increased cost up front.

Also odds are this thing will be a mish-mash of GM or Ford parts bin with just the body and body specific stuff being different (like the LLV, or that handicapped van AM general used to make). So they may very well be buying a "customized Ford Transit" where it just so happens the body is also custom.