I guess this theory has the right to live, but it kind of contradict with the info we already aquire. First of all, what about that mysterious place with lots of cages, in one of which MC was before "awakening"? Are you implying that he somehow created giantesses and then put himself in prison? Or maybe his creations did that? (Ancient Greece mythology reference lol)
But why? Why would they want to punish someone who made them the most supperior beings in the world? Maybe I just get that wrong. Second, if giantesses were created by MC, how exactly did that happen? On one bright sunny day the first ever giantess simply walked out of the forest to a human village and humans were like: "wtf?". Or did they appear before humans? But they need them to breed, so it would mean an extinction. We have 0 info about world past so there are too many unanswered questions for this theory to be solid.
Regarding the cages thing: It is a place to put most extraordinary/interesting people and keep them away from the world.
Regarding the punishment: 1. It is in the nature of the created to rebel against their creators.
2. Superior does not mean infallible, and the most dangerous being that could pose a threat to their lives is none other than the creator itself. They had every reason to fear him.
Regarding the Giantessess: They were most likely created through experiments, a combination of science, magic and other mystical arts most likely, on multiple individuals that were released across the world,in an effort to stop the numerous conflicts across the globe and bring a state of 'relative' peace.
Regarding why the Giantessess cant breed: The MC most likely managed to modify the Giantess gene and physiology either at the start or after they started believing themselves to be gods, ensuring that they could never wipe out the tinier people without risking extinction themselves.
Why the MC is still alive: If he/she/they(ambiguous) was responsible for the creation and modification of their race, then the giantessess must've also considered him too useful to throw away/kill, so they put him in a cage, to be released and acessed anytime they felt a threat to their race.
The MC's amnesia: We can only speculate on this, but one potential theory is that the MC has simply forgotten everything except their name during his stay of millenia's in limbo.