Ian Bremmer discusses the World In (more than) 60 Seconds:

Why hasn't Putin congratulated Biden yet?

There's no really good reason at this point. Pretty much every leader around the world has given the nod. As you know, Trump has not in any way conceded at this point. He may never. I suppose, at some point Putin may decide that he doesn't need to formally congratulate Biden. I mean, it's not like we're friends, right? The United States and Russia has a directly confrontational relationship, unlike the US and China, where there is a lot of interdependence, particularly economically between the two countries. That's not true with the US and Russia. You have virtually no trust and very little engagement. I will say that the Biden administration will be interested in re-entering the Open Skies agreement that we just left with the Russians, even though we're now decommissioning the spy plane, so it may be hard for the Americans and selling them for scrap, so it may be difficult to get back in and the intermediate nuclear forces agreement and new start.