I've actually wondered why almost all mice and keyboard come with fixed cables instead of mini/micro-b connector. I guess it saves couple of cents in part costs.
A lot of them are USB low-speed devices, especially the older ones, and the USB spec requires low-speed devices to have fixed cables because they have stricter cable-length restrictions than other devices.
My Happy Hacker (pro jp) keyboard doesn't have a fixed cable, which is a nice bonus, making it even more portable. Unfortunately the HHKB lite versions seem to have fixed cables :(
