Some of the bigger SH Tin robots were re issued, because Metal House still existed till they closed their doors some years ago. They had the machines and molds to make the Horikawa parts for the bigger robots, which we all know. I argue that the smaller Horikawa robots were not made by Metal House ( former Marumiya). I don't know, if GERNOT has more informations about other companies, which produced for Horikawa. But I guess, they have closed their gates long time ago...
The second thing is, that for a fabrication, you don't need only the molds. You Need an assortment of machines and of course people with the "know how". and you need other companies to produce other parts and the mechanism and so on. Not to speak about the lithoes on the tin parts, which had also been produced by different companies, which do no longer exist. Way too ornate & expensive only to produce some hundreds of pieces.
Interesting points and it could well be that MH didn't make the smaller plastic ones. I don't think a lack of lithos on those would be too bad though as most of them didn't have much anyway.
If thet do exist though and MH hasn't got them I surprised that whoever has them didn't see MH were selling the larger all tin ones and tried selling a few. As Dirk says the costs of plastic toys are high but already having the moulds would probably cut out a lot of the costs involved in tooling up a robot completely from scratch. After all other plastic toys have been reissued like the old Marx Daleks for instance.