The gen 1 mouse had removable batteries connected to a door on the bottom.
The gen 2 mouse went with a rechargable battery, but they didn’t want to retool the factories making the top shell, so they couldn’t put the charging port anywhere other than the bottom part they were redesigning.
Just charge the mouse overnight when you’re not using it. Also the charge cable disables the sensor so you can’t fix this “design feature” by modifying the mouse. Why not buy the desktop trackpad if you really need an input device when your mouse is charging?