Then computers got faster, much much faster. It became possible to redraw the whole UI from state every frame without it being a significant cost.
At the same time retained user interfaces managed to become more and more costly to do just about anything. I don't think for any particular reason other than computers were fast and they didn't need to do much better.
I find it really odd that there are user interfaces that take longer to rearrange their items than it takes for the same CPU to RayTrace a scene covering the same amount of screen area.