Probably just because it is harder to implement, the CSS would be an optimisation of a specific case on top of the general code. Classic cost (in time and overall complexity) vs performance tradeoff.
Basically line 182 plus the calls to redraw:
gamma -= 0.003;
It might into trouble because rotating a rectangular <img> area varies it in height and width [1], but I would investigate animating a ‘rotate’ transformation. https://developer.mozilla.org/en-US/docs/Web/CSS/transform-f....
[1] seems to have a demo without that problem, so it seems this can be made to work https://imagekit.io/blog/image-rotation-html-and-css-imageki...
It doesn't have a satellite image option, but that would arguably be simpler than drawing continents like it does now. It uses a seldom-redrawn base canvas (menu -> debug > show 2d texture) then the discs are drawn with a simple shader.