If you look at the front edge of the Magic Mouse, you will see that it is very thin. The top service some down to just a few millimeters above the desk surface. There isn’t room for a port there. Apple would have to redesign the mouse and they clearly don’t want to do that.

If you talk to people who use the Magic Mouse, they will mostly tell you that it is a non-issue. The mouse goes months on a charge and charges quickly. Just plug it in and grab a cup of coffee.