Indeed, bringing your own keyboard and mouse if necessary should be a nobrainer. Though having the employer pay for those things would be even better, but it’s not always the case.
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.