The reason I ask this question was as follows…. I was told that the Gohonzon I was given at Nichiren Shu was a silkscreen of the last Gohonzon that Nichiren himself made; the Shutei Gohonzon. I was surprised to go to Soka Gakkai and see a Gohonzon that was not made by Nichiren. Unlike other schools of Nichiren Buddhism, Soka Gakkai teaches that Nichiren was The Buddha of our age. Guess I had figured that Soka Gakkai would therefore be doubly inclined to have a Gohonzon that was by Nichiren.
There has been a long tradition of wood block reproduction of gohonzon, as well as carving reproductions into wood (the Shoshu so-called Dai-Gohonzon is an example). Other people inscribed Gohonzon also, a practice that started during Nichiren's life. Its a matter of practicality. The volume of Gohonzon that were required during Soka Gakkai's expansion would have been impossible to have them all inscribed by hand. I think part of the reason almost all Gohonzon are printed these days is primarily practical - volume required and the lack of people with both calligraphic skill and religious training to carry it out. There may also be sectarian limitations on authority to do so. Its also an expression of ideology - some Gohonzon by Nichiren have different composition, reflecting emphasis on one aspect or another. This was often done by Nichiren to address the specific spiritual needs of the recipient, and then the various schools select one or the other to express their teachings. I have a reproduction of a Nikko Shonin inscribed Gohonzon. Kitayama Hommonji used to issue Nichiren Gohonzon, but changed to emphasize the Nikko lineage. Its a subject that is probably worth some scholarly study.