The same reason Inoue is fixated over misunderstandings and rivers. When you are being worked like a machine and writing like 50+ scripts per year, you are going to be recycling a lot of ideas. Remember all Sanjo has been doing is toku since the start of Fourze: secondary write on Fourze -> preproduction and planning on Kyoryuger (after Fourze) -> writing all the episodes of Kyoryuger -> preproduction/planning for Drive (after Kyoryuger) -> main writer on Driver.
It's not like Kobayashi, Urobuchi or Nakashima where they do other stuff like novels, anime or VNs. All he has done is toku for like two years (barring that fiasco with Digimon). And that stint on Kyoryuger probably did to him what writing Faiz did to Inoue. Because it might be a creative challenge to write everything, but I am almost certain that sort of schedule will burnout the writer creatively