Not to diminish the art required to make resin molds, the high temperatures necessary to mold many plastics requires metal molds that won't warp during continued temperature changes. Simpler plaster molds are sufficient to withstand the heat produced by the exothermic reaction of curing resin. Although I'm sure the doll companies use something more durable than plaster for their molds, but still a material easier to work than steel or bronze.
Zirconmermaid, here on this board, has extensive experience with molding resin parts. She can speak to the relative simplicity of the process. She can also tell you how tricky it can be sometimes.
One thing to remember is that Obitsu is primarily a plastic manufacturer. They will take just about anyone's designs and produce their plastic toy products. Being just the manufacturer means they have less financial risk. When approached with a project they can say that will cost XX dollars and ask to be paid up front before manufacturing begins and all their costs are covered. With their doll line, they incur the financial burden of paying for everything before any product is sold. And if it doesn't sell, it's a financial loss. So there is less financial impetus for them to come out with new designs every few months. Let other companies (Parabox, Yamato, Visuadoll, Volks, Seedring- all companies Obitsu has made plastic/vinyl BJDs for) take the financial risk.
Here's a link to the Obitsu site and a quick step by step of their mold making process.
http://www.obitsu.co.jp/eng/seisaku/fr_seisaku.html