Because they need to be hard enough to resist deformation, yet resilient enough not to crack under cyclic load.
An analog would be "Why are tools made out of chrome-moly steel?" -well, because, if they were made out of mild steel they'd deform / nick too easily and, if they were made out of cast iron, they'd break if we used too much force or eventually after repeated use. So they're made out of an alloy which combines the properties we want.
I actually don't know enough about bearings to give a specific example, but when you think about, e.g., the entire weight of your car sitting on just 4 bearings, it's not hard to imagine the material they're made out of needing to be tough, yet not brittle.