In order to make a surface properly flat you need to have the cleave surface line up with crystal boundaries, but crystals are randomly oriented in three dimens

In order to make a surface properly flat you need to have the cleave surface line up with crystal boundaries, but crystals are randomly oriented in three dimensions meaning the surface can't be truly flat. And that's even assuming the solid you're working with is crystalline of the sort that can do this. Many materials are alloys meaning that cold welding would be further difficult.