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.
That's the problem I suppose but I think quite a few collectors would buy them seeing as they're well known robots. I think if MH could sell the Space Giants and Space Evil robots they could sell some of these Again there's quite a few of them about but it's very hard to get like new ones even at a high price. It's a shame they were made so the chrome parts tarnish so easily on them.
Yes a lot of the Japanese tin robots have been reissued or very close copies such as the Space Giant and those American plastic robots as you say but for some reason the Japanese plastic robots haven't been reissued. I wonder why?
Maybe it is because the moulds no longer exist or maybe companies like Metalhouse only thought people were interested in the tin robots, which isn't the case.