I agree with @Cornettoninja. Putin does because he can. He has the resources, has priced in sanctions, knows China is looking at Taiwan. He's not threatened by money or by the Russian people suffering. He's not threatened by NATO because he believes it doesn't have an appetite for conflict (neither do I to be honest).
He might frame it righting historical wrongs - uniting that which was separated etc - but that is to provide a story to stand behind.
Personally I think he'll shatter Ukraine whilst we watch and then he (or his successor) will continue to chip away at the Balkan countries.
Keep - oligarchs money is very important. You can deny it all you like but if Russian money can buy political influence in the UK (and potentially stymie our reactions to Russian aggression) it is all of business.