(I have a couple of colleagues who brought their own keyboards. We'd buy a decent keyboard for anyone who wants one, but they already had them from previous jobs with people who wouldn't do this.)
I, for example, absolutely hate mechanical keyboards with those tall keys, as I get hand/wrist/elbow pain from using them. And the mouse has to be as weightless as possible.
Considering companies usually have to decide on only 1 or 2 options to negotiate buying in bulk, it's hard to offer multiple options. Unless they give you like $100 when you start to buy whatever you want.