If we can map f13-f24 and people come out with programs for their mouse’s that also support f13-f24 you now have 12 keys you can program for macros, right now windows supports F13-F24 and has it in its code. The common issue with programmable mice is that you often have to set a macro to each button with a combination of key presses to achieve your bind but that can screw things up in games and may not work, setting up a single key is much more simplistic and allows the user the ability to use a key that is not used for typing or macros to achieve a key bind. Take for instance Broadcast Shield if I had it on F13 and bound my mouse to broadcast F13 when I press whatever key I bind it to than I would achieve a very easy setup.
Windows has a set number of programmable keys, you cannot add to it or take away from it and that is why we use macros and combination key presses. But if we expand our compatibility to include what is already there we can allow yourself to use keys that aren’t used for anything except what you want them to be.