Grendizer is after Mazinger and Great Mazinger, Koji was a high school kid in Mazinger who later went off to NASA which is where he learned to make the TFO before he joins Duke in his fight. This Koji is the same Koji from Mazinger Z and who later showed up in Great Mazinger. These three series are in continuity, everything after like Mazinkaiser is a alternate continuity.

There is a very short flashback of Mazinger, still pictures style, in the first scene of the first episode.

The most common answer to the reason behind the absence of the Mazingers is simply that Go Nagai just didn't want to make another Mazinger series at that time. Even Koji was a compromise to please Toei. In-story, there's no real explanation as to why he just doesn't grab a Mazinger (which were still in working order) to help other than that he just doesn't feel like it, as far as I remember.

I think they were trying to distance Grendizer from Mazinger, besides a flashback to Koji fighting in Mazinger in the first episode and then Boss showing up later the Mazinger Z connection isn't mentioned much. I guess they wanted to focus on Duke and Grendizer while Koji was a supporting character. Koji was mainly used to draw interest to Grendizer which really has zero connection to the Mazingers, after that it stayed focused on Grendizer and it's story.