It was very likely a designers decision. It forces the use the use case they wanted; wireless mice should be used wirelessly. I would bet they fought marketing and management to get this on the final product.

Marketing would want the mouse they can advertise as being useable with and wireless. Female ports are easier to mount and manufacture with they have depth to set the socket. So a plug on the front is much cheaper and easier to manufacture.

The fact the charging cable doesn’t get used in motion means it will last longer and you wouldn’t have people useing fraying cables on the front of their mouse.