Difference between states and electoral districts. Germany’s states are still unchanged from the 1949 West republic and 1990 addition of the East German states (Berlin did have a failed referendum to merge with Brandenburg). The U.S. constitution requires the consent of states to merge or spilt them
Plus, const. forbids depriving a state of statehood hat does not thrive. It would of course take an amendment to redraw the states. But as the SCOTUS is pretty close to just simply erasing certain amendments, a Dem president could simply order this. Using "emergency" powers. The new world is here.
France and the UK are also not federations unlike Germany and the U.S, so the regions are creations of the unitary government that can be changed by more simple legislation. This had led to some issues where the UK parliament has vetoed laws from the Scottish parliament.