Provide some evidence - go on do it.

They can't because it's an ongoing national security investigation so the only evidence is whatevers being passed around to the international community and enough of the IC have decided that Russia is or is very likely to be responsible based on it. When the investigation is complete, maybe you'll get the proof you want, but there is something going around that is clearly convincing enough that even the Baltic states are taking action.

Also having nuclear weapons doesn't mean you get to act like a twat and the world cowers around you. This is the point of MAD. There's a million and one other diplomatic, political, and economic escalatory options before war is even on the table.