It was a rhetorical question and the answer was: Because these are all male sexual fantasies that have a subtextual interest in treating women as livestock that worships you, and there is no symmetry in these fantasies between men and women where polyandry (or a more gender-equal polycule) could even be conceivable.

There are totally “reverse-harem” examples, but they are almost all targeting a female audience and, while I think those deserve criticism too, I think it’s not quite as bad or objectifying, it rarely has the same subtext of ownership except as a gag. I agree that male-targeted straight polyandry is going to just be cuckoldry fetish stuff most of the time even in the rare case it appears at all.