Mini-splits do exist in window-mountable form, although they haven't really caught on (yet). Technology Connections covers alternatives like that at the end of his portable ac episode https://youtu.be/_-mBeYC2KGc

I thought it was just that it was too difficult to fit all the parts efficiently in such a small box

When Tesla first publicized 3d printing the “octovalve” as a piece to complex to manufacture traditionally, the claim was that it made the heat pump much more compact. There was even a rumor going around that they might use it to enter the home heat pump market. Their work in making heat pumps compact enough for cars could really make a difference, however I image home stuff is much more price sensitive

From my understanding is a heat pump needs to be fully outside of the house.