Part of the problem is expectations. No one was comitting troops prior to the invasion, and the possible commitment of troops was one of the reasons Putin claimed he developed this situation to begin with, so it would have made a possibly avoidable situation immediately unavoidable. (Obviously he just went ahead anyway, hindsight.)
As such it's even more difficult to commit troops now, as Russia committed to its strategy with the information it had, and thus it would be taking an extremely large gamble at massive cost to the world.
However, troops are committed to various NATO members with a reinforcement of more. There is also a clear expectation that they will be defended should Russian aggression expand. Putin would have to willingly decide to bear this cost should he attack one of those nations, and its consequences. That wasn't the case at the time for the Ukraine.
Responding to erratic behavior with erratic behavior does not lead to stability. It's a moral crusade, which you could argue is worth the possibility of World War 3.
Imagine a bet in which nothing is exchanged if NATO has not gotten involved five years from now, but if they get involved and fewer than 1 million die within the next five years, you give me 100 €, but if they get involved and more than 1 million die within the next five years, I give you 500 €, would that seem like a fair bet to you?
(Die refers to as part of this conflict, e.g. using Wikipedia for verification.)
