Why is focus in a straight line when lenses are round?

I have been learning how to use a macro lens and I have finally got the hang of it and have been doing lots of focus stacking for my jewellery work, but is occluded to me....On all the images the area in focus is in a horizontal line, but the lens is round. It might seem more intuitive for the focus to be in a circle, how comes it isn’t? via /r/photography https://ift.tt/3ijobq9