I don't know either but I'm very much wonder, just like you, why this seems not possible to do. I'm also glad there are more intelligent people that noticed how good it was to have the throttle on the mouse wheel.
Come on 1C, please do a quick fix so we're able to map mouse wheel!
It's much wanted, shouldnt' take much time and it will be truly appreciated!
(Unless someone knows how to do this already of course.)