Consider trying to follow along with this app [0] without the transitions. Without them you'd experience much more cognitive load to staying oriented.
The key here is that animations happen outside the foveal area. Our vision is tuned to be extremely sensitive to motion and changes _outside_ the foveal area. So when something moves at the corner of your vision, it distracts your attention from your current focus.
This makes a lot of "modern" UI literally anti-productive. It actively _slows_ _down_ people and increases cognitive load.